Image encoding/decoding method and apparatus, and recording medium storing bitstream

ABSTRACT

An image decoding method is disclosed in the present specification. A method of decoding an image, the method may comprises determining whether to perform a context update for a first syntax element of a current block, updating, on the basis of the determination, a context for entropy decoding of the first syntax element and generating, on the basis of the updated context, a bin for the first syntax element, and wherein whether to perform the context update is determined on the basis of the number of pre-decoded predetermined syntax elements for the current block.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forencoding/decoding an image, and a recording medium for storing abitstream. More particularly, the present invention relates to a methodand an apparatus for encoding/decoding an image on the basis of anentropy coding, and a recording medium for storing a bitstream.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images, haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques arerequired for higher-resolution and higher-quality images.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; a transform and quantization technique for compressing energyof a residual signal; an entropy encoding technique of assigning a shortcode to a value with a high appearance frequency and assigning a longcode to a value with a low appearance frequency, etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

DISCLOSURE Technical Problem

An objective of the present invention is to provide an imageencoding/decoding method and apparatus capable of improving compressionefficiency, and a recording medium in which a bitstream generated by themethod or apparatus is stored.

Another objective of the present invention is to provide an imageencoding/decoding method and apparatus capable of improving compressionefficiency by using an entropy coding and a recording medium in which abitstream generated by the method or apparatus is stored.

Another objective of the present invention is to provide an imageencoding/decoding method and apparatus capable of improving throughputby using an entropy coding and a recording medium in which a bitstreamgenerated by the method or apparatus is stored.

Technical Solution

According to the present invention, image decoding method comprisesdetermining whether to perform a context update for a first syntaxelement of a current block, updating, on the basis of the determination,a context for entropy decoding of the first syntax element andgenerating, on the basis of the updated context, a bin for the firstsyntax element, wherein whether to perform the context update isdetermined on the basis of the number of pre-decoded predeterminedsyntax elements for the current block.

Wherein when the number of the pre-decoded predetermined syntax elementsis equal to or less than a preset value, the context update for thefirst syntax element is performed.

Wherein the preset value is determined on the basis of a size of thecurrent block.

Wherein only when the current block is in a transform skip mode, thecontext update is performed.

Wherein the first syntax element is a syntax element related to aresidual signal of the current block.

Wherein the first syntax element is the syntax element indicating a signof a quantized level.

Wherein the first syntax element is coeff_sign_flag.

Wherein the predetermined syntax element is at least one of residualsignal-related syntax elements.

Wherein the predetermined syntax elements are coeff_sign_flag,sig_coeff_flag, abs_level_gtx_flag, and par_level_flag.

According to the present invention, an image encoding method comprisesgenerating a bin for a first syntax element of a current block bybinarizing the first syntax element, determining whether to perform acontext update for the first syntax element, updating, on the basis ofthe determination, a context for entropy encoding of the first syntaxelement and generating a bitstream for the current block by using theupdated context and the generated bin, wherein whether to perform thecontext update is determined on the basis of the number of pre-encodedpredetermined syntax elements for the current block.

Wherein when the number of the pre-encoded predetermined syntax elementsis equal to or less than a preset value, the context update for thefirst syntax element is performed.

Wherein the preset value is determined on the basis of a size of thecurrent block.

Wherein only when the current block is in a transform skip mode, thecontext update is performed.

Wherein the first syntax element is a syntax element related to aresidual signal of the current block.

Wherein the first syntax element is the syntax element indicating a signof a quantized level.

Wherein the first syntax element is coeff_sign_flag.

Wherein the predetermined syntax element is at least one of residualsignal-related syntax elements.

Wherein the predetermined syntax elements are coeff_sign_flag,sig_coeff_flag, abs_level_gtx_flag, and par_level_flag.

According to a present invention, a computer-readable recording mediumstoring a bitstream that is received by an apparatus for decoding animage and is used to reconstruct a current block included in a currentpicture, wherein the bitstream is generated by a method of encoding animage, the method includes generating a bin for a first syntax elementof the current block by binarizing the first syntax element, determiningwhether to perform a context update for the first syntax element,updating, on the basis of the determination, a context for entropyencoding of the first syntax element and generating a bitstream by usingthe updated context and the generated bin, wherein whether to performthe context update is determined on the basis of the number ofpre-encoded predetermined syntax elements for the current block.

Advantageous Effects

According to the present invention, it is possible to provide an imageencoding/decoding method and apparatus capable of improving compressionefficiency and to provide a recording medium in which a bitstreamgenerated by the method or apparatus is stored.

In addition, according to the present invention, it is possible toprovide an image encoding/decoding method and apparatus capable ofimproving compression efficiency by using an entropy coding and arecording medium in which a bitstream generated by the method orapparatus is stored.

In addition, according to the present invention, it is possible toprovide an image encoding/decoding method and apparatus capable ofimproving throughput by using an entropy coding and a recording mediumin which a bitstream generated by the method or apparatus is stored.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view of a block diagram showing a configuration of anencoding apparatus to which the present invention is applied.

FIG. 2 is a view of a block diagram showing a configuration of adecoding apparatus to which the present invention is applied.

FIG. 3 is a view schematically showing a partition structure whenencoding and decoding an image.

FIG. 4 is a view showing an example of intra-prediction.

FIG. 5 is a view showing an example of inter-prediction.

FIG. 6 is a view showing an example of transform and quantization.

FIG. 7 is a view showing reference samples that are usable forintra-prediction.

FIG. 8 is a view showing an entropy decoding apparatus according to oneembodiment of the present invention.

FIG. 9 is a view showing an entropy encoding apparatus according to oneembodiment of the present invention.

FIG. 10 is a diagram illustrating an entropy encoding method and anentropy decoding method according to an embodiment of the presentinvention.

FIG. 11 is a diagram illustrating an entropy encoding method and anentropy decoding method according to another embodiment of the presentinvention.

FIG. 12 is another diagram illustrating an entropy encoding methodaccording to an embodiment of the present invention.

FIG. 13 is another diagram illustrating an entropy decoding methodaccording to an embodiment of the present invention.

FIG. 14 is a diagram illustrating a context and a binary stringaccording to an embodiment of the present invention.

FIG. 15 is a diagram illustrating a context update method using alook-up table according to an embodiment of the present invention.

FIG. 16 is another diagram illustrating a context update method using alook-up table according to an embodiment of the present invention.

FIG. 17 is a diagram illustrating a context update method using a linearprobability update model according to an embodiment of the presentinvention.

FIG. 18 is a diagram illustrating a context update method using alook-up table according to another embodiment of the present invention.

FIG. 19 is a diagram illustrating a context update method using a linearprobability update model according to another embodiment of the presentinvention.

FIG. 20 is a diagram illustrating a context update method using aprobability distribution model according to an embodiment of the presentinvention.

FIG. 21 is a diagram illustrating a method in which an entropy encodingor decoding process and a context update process are simultaneouslyperformed according to an embodiment of the present invention.

FIG. 22 is a diagram illustrating context initialization according to anembodiment of the present invention.

FIG. 23 is a diagram illustrating context initialization according toanother embodiment of the present invention.

FIG. 24 is a diagram illustrating a bitstream according to an embodimentof the present invention.

FIG. 25 is a diagram illustrating a bitstream according to anotherembodiment of the present invention.

BEST MODE

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, althoughthe exemplary embodiments can be construed as including allmodifications, equivalents, or substitutes in a technical concept and atechnical scope of the present invention. The similar reference numeralsrefer to the same or similar functions in various aspects. In thedrawings, the shapes and dimensions of elements may be exaggerated forclarity. In the following detailed description of the present invention,references are made to the accompanying drawings that show, by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to implement the present disclosure. Itshould be understood that various embodiments of the present disclosure,although different, are not necessarily mutually exclusive. For example,specific features, structures, and characteristics described herein, inconnection with one embodiment, may be implemented within otherembodiments without departing from the spirit and scope of the presentdisclosure. In addition, it should be understood that the location orarrangement of individual elements within each disclosed embodiment maybe modified without departing from the spirit and scope of the presentdisclosure. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present disclosure isdefined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to what the claims claim.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

Furthermore, constitutional parts shown in the embodiments of thepresent invention are independently shown so as to representcharacteristic functions different from each other. Thus, it does notmean that each constitutional part is constituted in a constitutionalunit of separated hardware or software. In other words, eachconstitutional part includes each of enumerated constitutional parts forconvenience. Thus, at least two constitutional parts of eachconstitutional part may be combined to form one constitutional part orone constitutional part may be divided into a plurality ofconstitutional parts to perform each function. The embodiment where eachconstitutional part is combined and the embodiment where oneconstitutional part is divided are also included in the scope of thepresent invention, if not departing from the essence of the presentinvention.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded. In other words, when a specific element is referred to as being“included”, elements other than the corresponding element are notexcluded, but additional elements may be included in embodiments of thepresent invention or the scope of the present invention.

In addition, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In describingexemplary embodiments of the present invention, well-known functions orconstructions will not be described in detail since they mayunnecessarily obscure the understanding of the present invention. Thesame constituent elements in the drawings are denoted by the samereference numerals, and a repeated description of the same elements willbe omitted.

Hereinafter, an image may mean a picture configuring a video, or maymean the video itself. For example, “encoding or decoding or both of animage” may mean “encoding or decoding or both of a moving picture”, andmay mean “encoding or decoding or both of one image among images of amoving picture.”

Hereinafter, terms “moving picture” and “video” may be used as the samemeaning and be replaced with each other.

Hereinafter, a target image may be an encoding target image which is atarget of encoding and/or a decoding target image which is a target ofdecoding. Also, a target image may be an input image inputted to anencoding apparatus, and an input image inputted to a decoding apparatus.Here, a target image may have the same meaning with the current image.

Hereinafter, terms “image”, “picture, “frame” and “screen” may be usedas the same meaning and be replaced with each other.

Hereinafter, a target block may be an encoding target block which is atarget of encoding and/or a decoding target block which is a target ofdecoding. Also, a target block may be the current block which is atarget of current encoding and/or decoding. For example, terms “targetblock” and “current block” may be used as the same meaning and bereplaced with each other.

Hereinafter, terms “block” and “unit” may be used as the same meaningand be replaced with each other. Or a “block” may represent a specificunit.

Hereinafter, terms “region” and “segment” may be replaced with eachother.

Hereinafter, a specific signal may be a signal representing a specificblock. For example, an original signal may be a signal representing atarget block. A prediction signal may be a signal representing aprediction block. A residual signal may be a signal representing aresidual block.

In embodiments, each of specific information, data, flag, index, elementand attribute, etc. may have a value. A value of information, data,flag, index, element and attribute equal to “0” may represent a logicalfalse or the first predefined value. In other words, a value “0”, afalse, a logical false and the first predefined value may be replacedwith each other. A value of information, data, flag, index, element andattribute equal to “1” may represent a logical true or the secondpredefined value. In other words, a value “1”, a true, a logical trueand the second predefined value may be replaced with each other.

When a variable i or j is used for representing a column, a row or anindex, a value of i may be an integer equal to or greater than 0, orequal to or greater than 1. That is, the column, the row, the index,etc. may be counted from 0 or may be counted from 1.

Description of Terms

Encoder: means an apparatus performing encoding. That is, means anencoding apparatus.

Decoder: means an apparatus performing decoding. That is, means andecoding apparatus.

Block: is an M×N array of a sample. Herein, M and N may mean positiveintegers, and the block may mean a sample array of a two-dimensionalform. The block may refer to a unit. A current block my mean an encodingtarget block that becomes a target when encoding, or a decoding targetblock that becomes a target when decoding. In addition, the currentblock may be at least one of an encode block, a prediction block, aresidual block, and a transform block.

Sample: is a basic unit constituting a block. It may be expressed as avalue from 0 to 2Bd−1 according to a bit depth (Bd). In the presentinvention, the sample may be used as a meaning of a pixel. That is, asample, a pel, a pixel may have the same meaning with each other.

Unit: may refer to an encoding and decoding unit. When encoding anddecoding an image, the unit may be a region generated by partitioning asingle image. In addition, the unit may mean a subdivided unit when asingle image is partitioned into subdivided units during encoding ordecoding. That is, an image may be partitioned into a plurality ofunits. When encoding and decoding an image, a predetermined process foreach unit may be performed. A single unit may be partitioned intosub-units that have sizes smaller than the size of the unit. Dependingon functions, the unit may mean a block, a macroblock, a coding treeunit, a code tree block, a coding unit, a coding block), a predictionunit, a prediction block, a residual unit), a residual block, atransform unit, a transform block, etc. In addition, in order todistinguish a unit from a block, the unit may include a luma componentblock, a chroma component block associated with the luma componentblock, and a syntax element of each color component block. The unit mayhave various sizes and forms, and particularly, the form of the unit maybe a two-dimensional geometrical figure such as a square shape, arectangular shape, a trapezoid shape, a triangular shape, a pentagonalshape, etc. In addition, unit information may include at least one of aunit type indicating the coding unit, the prediction unit, the transformunit, etc., and a unit size, a unit depth, a sequence of encoding anddecoding of a unit, etc.

Coding Tree Unit: is configured with a single coding tree block of aluma component Y, and two coding tree blocks related to chromacomponents Cb and Cr. In addition, it may mean that including the blocksand a syntax element of each block. Each coding tree unit may bepartitioned by using at least one of a quad-tree partitioning method, abinary-tree partitioning method and ternary-tree partitioning method toconfigure a lower unit such as coding unit, prediction unit, transformunit, etc. It may be used as a term for designating a sample block thatbecomes a process unit when encoding/decoding an image as an inputimage. Here, the quad-tree may mean a quaternary-tree.

When the size of the coding block is within a predetermined range, thedivision is possible using only quad-tree partitioning. Here, thepredetermined range may be defined as at least one of a maximum size anda minimum size of a coding block in which the division is possible usingonly quad-tree partitioning. Information indicating a maximum/minimumsize of a coding block in which quad-tree partitioning is allowed may besignaled through a bitstream, and the information may be signaled in atleast one unit of a sequence, a picture parameter, a tile group, or aslice (segment). Alternatively, the maximum/minimum size of the codingblock may be a fixed size predetermined in the coder/decoder. Forexample, when the size of the coding block corresponds to 256×256 to64×64, the division is possible only using quad-tree partitioning.Alternatively, when the size of the coding block is larger than the sizeof the maximum conversion block, the division is possible only usingquad-tree partitioning. Herein, the block to be divided may be at leastone of a coding block and a transform block. In this case, informationindicating the division of the coded block (for example, split_flag) maybe a flag indicating whether or not to perform the quad-treepartitioning. When the size of the coding block falls within apredetermined range, the division is possible only using binary tree orternary tree partitioning. In this case, the above description of thequad-tree partitioning may be applied to binary tree partitioning orternary tree partitioning in the same manner.

Coding Tree Block: may be used as a term for designating any one of a Ycoding tree block, Cb coding tree block, and Cr coding tree block.

Neighbor Block: may mean a block adjacent to a current block. The blockadjacent to the current block may mean a block that comes into contactwith a boundary of the current block, or a block positioned within apredetermined distance from the current block. The neighbor block maymean a block adjacent to a vertex of the current block. Herein, theblock adjacent to the vertex of the current block may mean a blockvertically adjacent to a neighbor block that is horizontally adjacent tothe current block, or a block horizontally adjacent to a neighbor blockthat is vertically adjacent to the current block.

Reconstructed Neighbor block: may mean a neighbor block adjacent to acurrent block and which has been already spatially/temporally encoded ordecoded. Herein, the reconstructed neighbor block may mean areconstructed neighbor unit. A reconstructed spatial neighbor block maybe a block within a current picture and which has been alreadyreconstructed through encoding or decoding or both. A reconstructedtemporal neighbor block is a block at a corresponding position as thecurrent block of the current picture within a reference image, or aneighbor block thereof.

Unit Depth: may mean a partitioned degree of a unit. In a treestructure, the highest node(Root Node) may correspond to the first unitwhich is not partitioned. Also, the highest node may have the leastdepth value. In this case, the highest node may have a depth of level 0.A node having a depth of level 1 may represent a unit generated bypartitioning once the first unit. A node having a depth of level 2 mayrepresent a unit generated by partitioning twice the first unit. A nodehaving a depth of level n may represent a unit generated by partitioningn-times the first unit. A Leaf Node may be the lowest node and a nodewhich cannot be partitioned further. A depth of a Leaf Node may be themaximum level. For example, a predefined value of the maximum level maybe 3. A depth of a root node may be the lowest and a depth of a leafnode may be the deepest. In addition, when a unit is expressed as a treestructure, a level in which a unit is present may mean a unit depth.

Bitstream: may mean a bitstream including encoding image information.

Parameter Set: corresponds to header information among a configurationwithin a bitstream. At least one of a video parameter set, a sequenceparameter set, a picture parameter set, and an adaptation parameter setmay be included in a parameter set. In addition, a parameter set mayinclude a slice header, a tile group header, and tile headerinformation. The term “tile group” means a group of tiles and has thesame meaning as a slice.

The adaptation parameter set refers to a parameter set that can beshared and referred to by different pictures, subpictures, slices, tilegroups, tiles, or bricks. In addition, sub-pictures, slices, tilegroups, tiles, or bricks in a picture may refer to different adaptationparameter sets to use information in the different adaptation parametersets.

Regarding the adaptation parameter sets, sub-pictures, slices, tilegroups, tiles, or bricks in a picture may refer to different adaptationparameter sets by using identifiers of the respective adaptationparameter sets.

Regarding the adaptation parameter sets, slices, tile groups, tiles, orbricks in a sub-picture may refer to different adaptation parameter setsby using identifiers of the respective adaptation parameter sets.

Regarding the adaptation parameter sets, tiles or bricks in a slice mayrefer to different adaptation parameter sets by using identifiers of therespective adaptation parameter sets.

Regarding the adaptation parameter sets, bricks in a tile may refer todifferent adaptation parameter sets by using identifiers of therespective adaptation parameter sets.

The parameter set or header of a sub-picture may include information onan adaptation parameter set identifier. Thus, an adaptation parameterset corresponding to the adaptation parameter set identifier may be usedin the sub-picture.

The parameter set or header of a tile may include an adaption parameterset identifier so that an adaption parameter set corresponding to theadaption parameter set identifier may be used in the tile.

The header of a brick may include information on an adaptation parameterset identifier so that an adaptation parameter set corresponding to theadaptation parameter set identifier may be used in the brick.

The picture may be split into one or more tile rows and one or more tilecolumns.

The sub-picture in a picture may be split into one or more tile rows andone or more tile columns. The sub-picture may be a rectangular or squareregion in a picture and may include one or more CTUs. The sub-picturemay include at least one tile, brick, and/or slice.

The tile may be a rectangular or square region in a picture and mayinclude one or more CTUs. The tile may be split into one or more bricks.

The brick may refer to one or more CTU rows in a tile. The tile may besplit into one or more bricks, and each brick may have at least one CTUrow. A tile that is not split into two or more bricks may also mean abrick.

The slice may include one or more tiles in a picture and may include oneor more bricks in a tile.

Parsing: may mean determination of a value of a syntax element byperforming entropy decoding, or may mean the entropy decoding itself.

Symbol: may mean at least one of a syntax element, a coding parameter,and a transform coefficient value of an encoding/decoding target unit.In addition, the symbol may mean an entropy encoding target or anentropy decoding result.

Prediction Mode: may be information indicating a mode encoded/decodedwith intra prediction or a mode encoded/decoded with inter prediction.

Prediction Unit: may mean a basic unit when performing prediction suchas inter-prediction, intra-prediction, inter-compensation,intra-compensation, and motion compensation. A single prediction unitmay be partitioned into a plurality of partitions having a smaller size,or may be partitioned into a plurality of lower prediction units. Aplurality of partitions may be a basic unit in performing prediction orcompensation. A partition which is generated by dividing a predictionunit may also be a prediction unit.

Prediction Unit Partition: may mean a form obtained by partitioning aprediction unit.

Reference picture list may refer to a list including one or morereference pictures used for inter prediction or motion compensation.There are several types of usable reference picture lists, including LC(List combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3).

Inter prediction indicator may refer to a direction of inter prediction(unidirectional prediction, bidirectional prediction, etc.) of a currentblock. Alternatively, it may refer to the number of reference picturesused to generate a prediction block of a current block. Alternatively,it may refer to the number of prediction blocks used at the time ofperforming inter prediction or motion compensation on a current block.

Prediction list utilization flag indicates whether a prediction block isgenerated using at least one reference picture in a specific referencepicture list. An inter prediction indicator can be derived using aprediction list utilization flag, and conversely, a prediction listutilization flag can be derived using an inter prediction indicator. Forexample, when the prediction list utilization flag has a first value ofzero (0), it means that a reference picture in a reference picture listis not used to generate a prediction block. On the other hand, when theprediction list utilization flag has a second value of one (1), it meansthat a reference picture list is used to generate a prediction block.

Reference picture index may refer to an index indicating a specificreference picture in a reference picture list.

Reference picture may mean a reference picture which is referred to by aspecific block for the purposes of inter prediction or motioncompensation of the specific block. Alternatively, the reference picturemay be a picture including a reference block referred to by a currentblock for inter prediction or motion compensation. Hereinafter, theterms “reference picture” and “reference image” have the same meaningand can be interchangeably.

Motion vector may be a two-dimensional vector used for inter predictionor motion compensation. The motion vector may mean an offset between anencoding/decoding target block and a reference block. For example, (mvX,mvY) may represent a motion vector. Here, mvX may represent a horizontalcomponent and mvY may represent a vertical component.

Search range may be a two-dimensional region which is searched toretrieve a motion vector during inter prediction. For example, the sizeof the search range may be M×N. Here, M and N are both integers.

Motion vector candidate may refer to a prediction candidate block or amotion vector of the prediction candidate block when predicting a motionvector. In addition, a motion vector candidate may be included in amotion vector candidate list.

Motion vector candidate list may mean a list composed of one or moremotion vector candidates.

Motion vector candidate index may mean an indicator indicating a motionvector candidate in a motion vector candidate list. Alternatively, itmay be an index of a motion vector predictor.

Motion information may mean information including at least one of theitems including a motion vector, a reference picture index, an interprediction indicator, a prediction list utilization flag, referencepicture list information, a reference picture, a motion vectorcandidate, a motion vector candidate index, a merge candidate, and amerge index.

Merge candidate list may mean a list composed of one or more mergecandidates.

Merge candidate may mean a spatial merge candidate, a temporal mergecandidate, a combined merge candidate, a combined bi-predictive mergecandidate, or a zero merge candidate. The merge candidate may includemotion information such as an inter prediction indicator, a referencepicture index for each list, a motion vector, a prediction listutilization flag, and an inter prediction indicator.

Merge index may mean an indicator indicating a merge candidate in amerge candidate list. Alternatively, the merge index may indicate ablock from which a merge candidate has been derived, among reconstructedblocks spatially/temporally adjacent to a current block. Alternatively,the merge index may indicate at least one piece of motion information ofa merge candidate.

Transform Unit: may mean a basic unit when performing encoding/decodingsuch as transform, inverse-transform, quantization, dequantization,transform coefficient encoding/decoding of a residual signal. A singletransform unit may be partitioned into a plurality of lower-leveltransform units having a smaller size. Here,transformation/inverse-transformation may comprise at least one amongthe first transformation/the first inverse-transformation and the secondtransformation/the second inverse-transformation.

Scaling: may mean a process of multiplying a quantized level by afactor. A transform coefficient may be generated by scaling a quantizedlevel. The scaling also may be referred to as dequantization.

Quantization Parameter: may mean a value used when generating aquantized level using a transform coefficient during quantization. Thequantization parameter also may mean a value used when generating atransform coefficient by scaling a quantized level duringdequantization. The quantization parameter may be a value mapped on aquantization step size.

Delta Quantization Parameter: may mean a difference value between apredicted quantization parameter and a quantization parameter of anencoding/decoding target unit.

Scan: may mean a method of sequencing coefficients within a unit, ablock or a matrix. For example, changing a two-dimensional matrix ofcoefficients into a one-dimensional matrix may be referred to asscanning, and changing a one-dimensional matrix of coefficients into atwo-dimensional matrix may be referred to as scanning or inversescanning.

Transform Coefficient: may mean a coefficient value generated aftertransform is performed in an encoder. It may mean a coefficient valuegenerated after at least one of entropy decoding and dequantization isperformed in a decoder. A quantized level obtained by quantizing atransform coefficient or a residual signal, or a quantized transformcoefficient level also may fall within the meaning of the transformcoefficient.

Quantized Level: may mean a value generated by quantizing a transformcoefficient or a residual signal in an encoder. Alternatively, thequantized level may mean a value that is a dequantization target toundergo dequantization in a decoder. Similarly, a quantized transformcoefficient level that is a result of transform and quantization alsomay fall within the meaning of the quantized level.

Non-zero Transform Coefficient: may mean a transform coefficient havinga value other than zero, or a transform coefficient level or a quantizedlevel having a value other than zero.

Quantization Matrix: may mean a matrix used in a quantization process ora dequantization process performed to improve subjective or objectiveimage quality. The quantization matrix also may be referred to as ascaling list.

Quantization Matrix Coefficient: may mean each element within aquantization matrix. The quantization matrix coefficient also may bereferred to as a matrix coefficient.

Default Matrix: may mean a predetermined quantization matrixpreliminarily defined in an encoder or a decoder.

Non-default Matrix: may mean a quantization matrix that is notpreliminarily defined in an encoder or a decoder but is signaled by auser.

Statistic Value: a statistic value for at least one among a variable, acoding parameter, a constant value, etc. which have a computablespecific value may be one or more among an average value, a sum value, aweighted average value, a weighted sum value, the minimum value, themaximum value, the most frequent value, a median value, an interpolatedvalue of the corresponding specific values.

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatus,or an image encoding apparatus. A video may include at least one image.The encoding apparatus 100 may sequentially encode at least one image.

Referring to FIG. 1, the encoding apparatus 100 may include a motionprediction unit 111, a motion compensation unit 112, an intra-predictionunit 120, a switch 115, a subtractor 125, a transform unit 130, aquantization unit 140, an entropy encoding unit 150, a dequantizationunit 160, an inverse-transform unit 170, an adder 175, a filter unit180, and a reference picture buffer 190.

The encoding apparatus 100 may perform encoding of an input image byusing an intra mode or an inter mode or both. In addition, encodingapparatus 100 may generate a bitstream including encoded informationthrough encoding the input image, and output the generated bitstream.The generated bitstream may be stored in a computer readable recordingmedium, or may be streamed through a wired/wireless transmission medium.When an intra mode is used as a prediction mode, the switch 115 may beswitched to an intra. Alternatively, when an inter mode is used as aprediction mode, the switch 115 may be switched to an inter mode.Herein, the intra mode may mean an intra-prediction mode, and the intermode may mean an inter-prediction mode. The encoding apparatus 100 maygenerate a prediction block for an input block of the input image. Inaddition, the encoding apparatus 100 may encode a residual block using aresidual of the input block and the prediction block after theprediction block being generated. The input image may be called as acurrent image that is a current encoding target. The input block may becalled as a current block that is current encoding target, or as anencoding target block.

When a prediction mode is an intra mode, the intra-prediction unit 120may use a sample of a block that has been already encoded/decoded and isadjacent to a current block as a reference sample. The intra-predictionunit 120 may perform spatial prediction for the current block by using areference sample, or generate prediction samples of an input block byperforming spatial prediction. Herein, the intra prediction may meanintra-prediction,

When a prediction mode is an inter mode, the motion prediction unit 111may retrieve a region that best matches with an input block from areference image when performing motion prediction, and deduce a motionvector by using the retrieved region. In this case, a search region maybe used as the region. The reference image may be stored in thereference picture buffer 190. Here, when encoding/decoding for thereference image is performed, it may be stored in the reference picturebuffer 190.

The motion compensation unit 112 may generate a prediction block byperforming motion compensation for the current block using a motionvector. Herein, inter-prediction may mean inter-prediction or motioncompensation.

When the value of the motion vector is not an integer, the motionprediction unit 111 and the motion compensation unit 112 may generatethe prediction block by applying an interpolation filter to a partialregion of the reference picture. In order to perform inter-pictureprediction or motion compensation on a coding unit, it may be determinedthat which mode among a skip mode, a merge mode, an advanced motionvector prediction (AMVP) mode, and a current picture referring mode isused for motion prediction and motion compensation of a prediction unitincluded in the corresponding coding unit. Then, inter-pictureprediction or motion compensation may be differently performed dependingon the determined mode.

The subtractor 125 may generate a residual block by using a differenceof an input block and a prediction block. The residual block may becalled as a residual signal. The residual signal may mean a differencebetween an original signal and a prediction signal. In addition, theresidual signal may be a signal generated by transforming or quantizing,or transforming and quantizing a difference between the original signaland the prediction signal. The residual block may be a residual signalof a block unit.

The transform unit 130 may generate a transform coefficient byperforming transform of a residual block, and output the generatedtransform coefficient. Herein, the transform coefficient may be acoefficient value generated by performing transform of the residualblock. When a transform skip mode is applied, the transform unit 130 mayskip transform of the residual block.

A quantized level may be generated by applying quantization to thetransform coefficient or to the residual signal. Hereinafter, thequantized level may be also called as a transform coefficient inembodiments.

The quantization unit 140 may generate a quantized level by quantizingthe transform coefficient or the residual signal according to aparameter, and output the generated quantized level. Herein, thequantization unit 140 may quantize the transform coefficient by using aquantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingentropy encoding according to a probability distribution on valuescalculated by the quantization unit 140 or on coding parameter valuescalculated when performing encoding, and output the generated bitstream.The entropy encoding unit 150 may perform entropy encoding of sampleinformation of an image and information for decoding an image. Forexample, the information for decoding the image may include a syntaxelement.

When entropy encoding is applied, symbols are represented so that asmaller number of bits are assigned to a symbol having a high chance ofbeing generated and a larger number of bits are assigned to a symbolhaving a low chance of being generated, and thus, the size of bit streamfor symbols to be encoded may be decreased. The entropy encoding unit150 may use an encoding method for entropy encoding such as exponentialGolomb, context-adaptive variable length coding (CAVLC),context-adaptive binary arithmetic coding (CABAC), etc. For example, theentropy encoding unit 150 may perform entropy encoding by using avariable length coding/code (VLC) table. In addition, the entropyencoding unit 150 may deduce a binarization method of a target symboland a probability model of a target symbol/bin, and perform arithmeticcoding by using the deduced binarization method, and a context model.

In order to encode a transform coefficient level(quantized level), theentropy encoding unit 150 may change a two-dimensional block formcoefficient into a one-dimensional vector form by using a transformcoefficient scanning method.

A coding parameter may include information (flag, index, etc.) such assyntax element that is encoded in an encoder and signaled to a decoder,and information derived when performing encoding or decoding. The codingparameter may mean information required when encoding or decoding animage. For example, at least one value or a combination form of aunit/block size, a unit/block depth, unit/block partition information,unit/block shape, unit/block partition structure, whether to partitionof a quad-tree form, whether to partition of a binary-tree form, apartition direction of a binary-tree form (horizontal direction orvertical direction), a partition form of a binary-tree form (symmetricpartition or asymmetric partition), whether or not a current coding unitis partitioned by ternary tree partitioning, direction (horizontal orvertical direction) of the ternary tree partitioning, type (symmetric orasymmetric type) of the ternary tree partitioning, whether a currentcoding unit is partitioned by multi-type tree partitioning, direction(horizontal or vertical direction) of the multi-type three partitioning,type (symmetric or asymmetric type) of the multi-type tree partitioning,and a tree (binary tree or ternary tree) structure of the multi-typetree partitioning, a prediction mode(intra prediction or interprediction), a luma intra-prediction mode/direction, a chromaintra-prediction mode/direction, intra partition information, interpartition information, a coding block partition flag, a prediction blockpartition flag, a transform block partition flag, a reference samplefiltering method, a reference sample filter tab, a reference samplefilter coefficient, a prediction block filtering method, a predictionblock filter tap, a prediction block filter coefficient, a predictionblock boundary filtering method, a prediction block boundary filter tab,a prediction block boundary filter coefficient, an intra-predictionmode, an inter-prediction mode, motion information, a motion vector, amotion vector difference, a reference picture index, a inter-predictionangle, an inter-prediction indicator, a prediction list utilizationflag, a reference picture list, a reference picture, a motion vectorpredictor index, a motion vector predictor candidate, a motion vectorcandidate list, whether to use a merge mode, a merge index, a mergecandidate, a merge candidate list, whether to use a skip mode, aninterpolation filter type, an interpolation filter tab, an interpolationfilter coefficient, a motion vector size, a presentation accuracy of amotion vector, a transform type, a transform size, information ofwhether or not a primary(first) transform is used, information ofwhether or not a secondary transform is used, a primary transform index,a secondary transform index, information of whether or not a residualsignal is present, a coded block pattern, a coded block flag (CBF), aquantization parameter, a quantization parameter residue, a quantizationmatrix, whether to apply an intra loop filter, an intra loop filtercoefficient, an intra loop filter tab, an intra loop filter shape/form,whether to apply a deblocking filter, a deblocking filter coefficient, adeblocking filter tab, a deblocking filter strength, a deblocking filtershape/form, whether to apply an adaptive sample offset, an adaptivesample offset value, an adaptive sample offset category, an adaptivesample offset type, whether to apply an adaptive loop filter, anadaptive loop filter coefficient, an adaptive loop filter tab, anadaptive loop filter shape/form, a binarization/inverse-binarizationmethod, a context model determining method, a context model updatingmethod, whether to perform a regular mode, whether to perform a bypassmode, a context bin, a bypass bin, a significant coefficient flag, alast significant coefficient flag, a coded flag for a unit of acoefficient group, a position of the last significant coefficient, aflag for whether a value of a coefficient is larger than 1, a flag forwhether a value of a coefficient is larger than 2, a flag for whether avalue of a coefficient is larger than 3, information on a remainingcoefficient value, a sign information, a reconstructed luma sample, areconstructed chroma sample, a residual luma sample, a residual chromasample, a luma transform coefficient, a chroma transform coefficient, aquantized luma level, a quantized chroma level, a transform coefficientlevel scanning method, a size of a motion vector search area at adecoder side, a shape of a motion vector search area at a decoder side,a number of time of a motion vector search at a decoder side,information on a CTU size, information on a minimum block size,information on a maximum block size, information on a maximum blockdepth, information on a minimum block depth, an imagedisplaying/outputting sequence, slice identification information, aslice type, slice partition information, tile identificationinformation, a tile type, tile partition information, tile groupidentification information, a tile group type, tile group partitioninformation, a picture type, a bit depth of an input sample, a bit depthof a reconstruction sample, a bit depth of a residual sample, a bitdepth of a transform coefficient, a bit depth of a quantized level, andinformation on a luma signal or information on a chroma signal may beincluded in the coding parameter.

Herein, signaling the flag or index may mean that a corresponding flagor index is entropy encoded and included in a bitstream by an encoder,and may mean that the corresponding flag or index is entropy decodedfrom a bitstream by a decoder.

When the encoding apparatus 100 performs encoding throughinter-prediction, an encoded current image may be used as a referenceimage for another image that is processed afterwards. Accordingly, theencoding apparatus 100 may reconstruct or decode the encoded currentimage, or store the reconstructed or decoded image as a reference imagein reference picture buffer 190.

A quantized level may be dequantized in the dequantization unit 160, ormay be inverse-transformed in the inverse-transform unit 170. Adequantized or inverse-transformed coefficient or both may be added witha prediction block by the adder 175. By adding the dequantized orinverse-transformed coefficient or both with the prediction block, areconstructed block may be generated. Herein, the dequantized orinverse-transformed coefficient or both may mean a coefficient on whichat least one of dequantization and inverse-transform is performed, andmay mean a reconstructed residual block.

A reconstructed block may pass through the filter unit 180. The filterunit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to areconstructed sample, a reconstructed block or a reconstructed image.The filter unit 180 may be called as an in-loop filter.

The deblocking filter may remove block distortion generated inboundaries between blocks. In order to determine whether or not to applya deblocking filter, whether or not to apply a deblocking filter to acurrent block may be determined based samples included in several rowsor columns which are included in the block. When a deblocking filter isapplied to a block, another filter may be applied according to arequired deblocking filtering strength.

In order to compensate an encoding error, a proper offset value may beadded to a sample value by using a sample adaptive offset. The sampleadaptive offset may correct an offset of a deblocked image from anoriginal image by a sample unit. A method of partitioning samples of animage into a predetermined number of regions, determining a region towhich an offset is applied, and applying the offset to the determinedregion, or a method of applying an offset in consideration of edgeinformation on each sample may be used.

The adaptive loop filter may perform filtering based on a comparisonresult of the filtered reconstructed image and the original image.Samples included in an image may be partitioned into predeterminedgroups, a filter to be applied to each group may be determined, anddifferential filtering may be performed for each group. Information ofwhether or not to apply the ALF may be signaled by coding units (CUs),and a form and coefficient of the ALF to be applied to each block mayvary.

The reconstructed block or the reconstructed image having passed throughthe filter unit 180 may be stored in the reference picture buffer 190. Areconstructed block processed by the filter unit 180 may be a part of areference image. That is, a reference image is a reconstructed imagecomposed of reconstructed blocks processed by the filter unit 180. Thestored reference image may be used later in inter prediction or motioncompensation.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

A decoding apparatus 200 may a decoder, a video decoding apparatus, oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization unit 220, an inverse-transform unit230, an intra-prediction unit 240, a motion compensation unit 250, anadder 225, a filter unit 260, and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer readable recording medium, or may receivea bitstream that is streamed through a wired/wireless transmissionmedium. The decoding apparatus 200 may decode the bitstream by using anintra mode or an inter mode. In addition, the decoding apparatus 200 maygenerate a reconstructed image generated through decoding or a decodedimage, and output the reconstructed image or decoded image.

When a prediction mode used when decoding is an intra mode, a switch maybe switched to an intra. Alternatively, when a prediction mode used whendecoding is an inter mode, a switch may be switched to an inter mode.

The decoding apparatus 200 may obtain a reconstructed residual block bydecoding the input bitstream, and generate a prediction block. When thereconstructed residual block and the prediction block are obtained, thedecoding apparatus 200 may generate a reconstructed block that becomes adecoding target by adding the reconstructed residual block with theprediction block. The decoding target block may be called a currentblock.

The entropy decoding unit 210 may generate symbols by entropy decodingthe bitstream according to a probability distribution. The generatedsymbols may include a symbol of a quantized level form. Herein, anentropy decoding method may be an inverse-process of the entropyencoding method described above.

In order to decode a transform coefficient level(quantized level), theentropy decoding unit 210 may change a one-directional vector formcoefficient into a two-dimensional block form by using a transformcoefficient scanning method.

A quantized level may be dequantized in the dequantization unit 220, orinverse-transformed in the inverse-transform unit 230. The quantizedlevel may be a result of dequantizing or inverse-transforming or both,and may be generated as a reconstructed residual block. Herein, thedequantization unit 220 may apply a quantization matrix to the quantizedlevel.

When an intra mode is used, the intra-prediction unit 240 may generate aprediction block by performing, for the current block, spatialprediction that uses a sample value of a block adjacent to a decodingtarget block and which has been already decoded.

When an inter mode is used, the motion compensation unit 250 maygenerate a prediction block by performing, for the current block, motioncompensation that uses a motion vector and a reference image stored inthe reference picture buffer 270.

The adder 225 may generate a reconstructed block by adding thereconstructed residual block with the prediction block. The filter unit260 may apply at least one of a deblocking filter, a sample adaptiveoffset, and an adaptive loop filter to the reconstructed block orreconstructed image. The filter unit 260 may output the reconstructedimage. The reconstructed block or reconstructed image may be stored inthe reference picture buffer 270 and used when performinginter-prediction. A reconstructed block processed by the filter unit 260may be a part of a reference image. That is, a reference image is areconstructed image composed of reconstructed blocks processed by thefilter unit 260. The stored reference image may be used later in interprediction or motion compensation.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image. FIG. 3 schematically shows anexample of partitioning a single unit into a plurality of lower units.

In order to efficiently partition an image, when encoding and decoding,a coding unit (CU) may be used. The coding unit may be used as a basicunit when encoding/decoding the image. In addition, the coding unit maybe used as a unit for distinguishing an intra prediction mode and aninter prediction mode when encoding/decoding the image. The coding unitmay be a basic unit used for prediction, transform, quantization,inverse-transform, dequantization, or an encoding/decoding process of atransform coefficient.

Referring to FIG. 3, an image 300 is sequentially partitioned in alargest coding unit (LCU), and a LCU unit is determined as a partitionstructure. Herein, the LCU may be used in the same meaning as a codingtree unit (CTU). A unit partitioning may mean partitioning a blockassociated with to the unit. In block partition information, informationof a unit depth may be included. Depth information may represent anumber of times or a degree or both in which a unit is partitioned. Asingle unit may be partitioned into a plurality of lower level unitshierarchically associated with depth information based on a treestructure. In other words, a unit and a lower level unit generated bypartitioning the unit may correspond to a node and a child node of thenode, respectively. Each of partitioned lower unit may have depthinformation. Depth information may be information representing a size ofa CU, and may be stored in each CU. Unit depth represents times and/ordegrees related to partitioning a unit. Therefore, partitioninginformation of a lower-level unit may comprise information on a size ofthe lower-level unit.

A partition structure may mean a distribution of a coding unit (CU)within an LCU 310. Such a distribution may be determined according towhether or not to partition a single CU into a plurality (positiveinteger equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs.A horizontal size and a vertical size of the CU generated bypartitioning may respectively be half of a horizontal size and avertical size of the CU before partitioning, or may respectively havesizes smaller than a horizontal size and a vertical size beforepartitioning according to a number of times of partitioning. The CU maybe recursively partitioned into a plurality of CUs. By the recursivepartitioning, at least one among a height and a width of a CU afterpartitioning may decrease comparing with at least one among a height anda width of a CU before partitioning. Partitioning of the CU may berecursively performed until to a predefined depth or predefined size.For example, a depth of an LCU may be 0, and a depth of a smallestcoding unit (SCU) may be a predefined maximum depth. Herein, the LCU maybe a coding unit having a maximum coding unit size, and the SCU may be acoding unit having a minimum coding unit size as described above.Partitioning is started from the LCU 310, a CU depth increases by 1 as ahorizontal size or a vertical size or both of the CU decreases bypartitioning. For example, for each depth, a CU which is not partitionedmay have a size of 2N×2N. Also, in case of a CU which is partitioned, aCU with a size of 2N×2N may be partitioned into four CUs with a size ofN×N. A size of N may decrease to half as a depth increase by 1.

In addition, information whether or not the CU is partitioned may berepresented by using partition information of the CU. The partitioninformation may be 1-bit information. All CUs, except for a SCU, mayinclude partition information. For example, when a value of partitioninformation is a first value, the CU may not be partitioned, when avalue of partition information is a second value, the CU may bepartitioned

Referring to FIG. 3, an LCU having a depth 0 may be a 64×64 block. 0 maybe a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may bea maximum depth. A CU of a 32×32 block and a 16×16 block may berespectively represented as a depth 1 and a depth 2.

For example, when a single coding unit is partitioned into four codingunits, a horizontal size and a vertical size of the four partitionedcoding units may be a half size of a horizontal and vertical size of theCU before being partitioned. In one embodiment, when a coding unithaving a 32×32 size is partitioned into four coding units, each of thefour partitioned coding units may have a 16×16 size. When a singlecoding unit is partitioned into four coding units, it may be called thatthe coding unit may be partitioned into a quad-tree form.

For example, when one coding unit is partitioned into two sub-codingunits, the horizontal or vertical size (width or height) of each of thetwo sub-coding units may be half the horizontal or vertical size of theoriginal coding unit. For example, when a coding unit having a size of32×32 is vertically partitioned into two sub-coding units, each of thetwo sub-coding units may have a size of 16×32. For example, when acoding unit having a size of 8×32 is horizontally partitioned into twosub-coding units, each of the two sub-coding units may have a size of8×16. When one coding unit is partitioned into two sub-coding units, itcan be said that the coding unit is binary-partitioned or is partitionedby a binary tree partition structure.

For example, when one coding unit is partitioned into three sub-codingunits, the horizontal or vertical size of the coding unit can bepartitioned with a ratio of 1:2:1, thereby producing three sub-codingunits whose horizontal or vertical sizes are in a ratio of 1:2:1. Forexample, when a coding unit having a size of 16×32 is horizontallypartitioned into three sub-coding units, the three sub-coding units mayhave sizes of 16×8, 16×16, and 16×8 respectively, in the order from theuppermost to the lowermost sub-coding unit. For example, when a codingunit having a size of 32×32 is vertically split into three sub-codingunits, the three sub-coding units may have sizes of 8×32, 16×32, and8×32, respectively in the order from the left to the right sub-codingunit. When one coding unit is partitioned into three sub-coding units,it can be said that the coding unit is ternary-partitioned orpartitioned by a ternary tree partition structure.

In FIG. 3, a coding tree unit (CTU) 320 is an example of a CTU to whicha quad tree partition structure, a binary tree partition structure, anda ternary tree partition structure are all applied.

As described above, in order to partition the CTU, at least one of aquad tree partition structure, a binary tree partition structure, and aternary tree partition structure may be applied. Various tree partitionstructures may be sequentially applied to the CTU, according to apredetermined priority order. For example, the quad tree partitionstructure may be preferentially applied to the CTU. A coding unit thatcannot be partitioned any longer using a quad tree partition structuremay correspond to a leaf node of a quad tree. A coding unitcorresponding to a leaf node of a quad tree may serve as a root node ofa binary and/or ternary tree partition structure. That is, a coding unitcorresponding to a leaf node of a quad tree may be further partitionedby a binary tree partition structure or a ternary tree partitionstructure, or may not be further partitioned. Therefore, by preventing acoding block that results from binary tree partitioning or ternary treepartitioning of a coding unit corresponding to a leaf node of a quadtree from undergoing further quad tree partitioning, block partitioningand/or signaling of partition information can be effectively performed.

The fact that a coding unit corresponding to a node of a quad tree ispartitioned may be signaled using quad partition information. The quadpartition information having a first value (e.g., “1”) may indicate thata current coding unit is partitioned by the quad tree partitionstructure. The quad partition information having a second value (e.g.,“C”) may indicate that a current coding unit is not partitioned by thequad tree partition structure. The quad partition information may be aflag having a predetermined length (e.g., one bit).

There may not be a priority between the binary tree partitioning and theternary tree partitioning. That is, a coding unit corresponding to aleaf node of a quad tree may further undergo arbitrary partitioningamong the binary tree partitioning and the ternary tree partitioning. Inaddition, a coding unit generated through the binary tree partitioningor the ternary tree partitioning may undergo a further binary treepartitioning or a further ternary tree partitioning, or may not befurther partitioned.

A tree structure in which there is no priority among the binary treepartitioning and the ternary tree partitioning is referred to as amulti-type tree structure. A coding unit corresponding to a leaf node ofa quad tree may serve as a root node of a multi-type tree. Whether topartition a coding unit which corresponds to a node of a multi-type treemay be signaled using at least one of multi-type tree partitionindication information, partition direction information, and partitiontree information. For partitioning of a coding unit corresponding to anode of a multi-type tree, the multi-type tree partition indicationinformation, the partition direction, and the partition tree informationmay be sequentially signaled.

The multi-type tree partition indication information having a firstvalue (e.g., “1”) may indicate that a current coding unit is to undergoa multi-type tree partitioning. The multi-type tree partition indicationinformation having a second value (e.g., “0”) may indicate that acurrent coding unit is not to undergo a multi-type tree partitioning.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, the codingunit may include partition direction information. The partitiondirection information may indicate in which direction a current codingunit is to be partitioned for the multi-type tree partitioning. Thepartition direction information having a first value (e.g., “1”) mayindicate that a current coding unit is to be vertically partitioned. Thepartition direction information having a second value (e.g., “0”) mayindicate that a current coding unit is to be horizontally partitioned.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, thecurrent coding unit may include partition tree information. Thepartition tree information may indicate a tree partition structure whichis to be used for partitioning of a node of a multi-type tree. Thepartition tree information having a first value (e.g., “1”) may indicatethat a current coding unit is to be partitioned by a binary treepartition structure. The partition tree information having a secondvalue (e.g., “0”) may indicate that a current coding unit is to bepartitioned by a ternary tree partition structure.

The partition indication information, the partition tree information,and the partition direction information may each be a flag having apredetermined length (e.g., one bit).

At least any one of the quadtree partition indication information, themulti-type tree partition indication information, the partitiondirection information, and the partition tree information may be entropyencoded/decoded. For the entropy-encoding/decoding of those types ofinformation, information on a neighboring coding unit adjacent to thecurrent coding unit may be used. For example, there is a highprobability that the partition type (the partitioned or non-partitioned,the partition tree, and/or the partition direction) of a leftneighboring coding unit and/or an upper neighboring coding unit of acurrent coding unit is similar to that of the current coding unit.Therefore, context information for entropy encoding/decoding of theinformation on the current coding unit may be derived from theinformation on the neighboring coding units. The information on theneighboring coding units may include at least any one of quad partitioninformation, multi-type tree partition indication information, partitiondirection information, and partition tree information.

As another example, among binary tree partitioning and ternary treepartitioning, binary tree partitioning may be preferentially performed.That is, a current coding unit may primarily undergo binary treepartitioning, and then a coding unit corresponding to a leaf node of abinary tree may be set as a root node for ternary tree partitioning. Inthis case, neither quad tree partitioning nor binary tree partitioningmay not be performed on the coding unit corresponding to a node of aternary tree.

A coding unit that cannot be partitioned by a quad tree partitionstructure, a binary tree partition structure, and/or a ternary treepartition structure becomes a basic unit for coding, prediction and/ortransformation. That is, the coding unit cannot be further partitionedfor prediction and/or transformation. Therefore, the partition structureinformation and the partition information used for partitioning a codingunit into prediction units and/or transformation units may not bepresent in a bit stream.

However, when the size of a coding unit (i.e., a basic unit forpartitioning) is larger than the size of a maximum transformation block,the coding unit may be recursively partitioned until the size of thecoding unit is reduced to be equal to or smaller than the size of themaximum transformation block. For example, when the size of a codingunit is 64×64 and when the size of a maximum transformation block is32×32, the coding unit may be partitioned into four 32×32 blocks fortransformation. For example, when the size of a coding unit is 32×64 andthe size of a maximum transformation block is 32×32, the coding unit maybe partitioned into two 32×32 blocks for the transformation. In thiscase, the partitioning of the coding unit for transformation is notsignaled separately, and may be determined through comparison betweenthe horizontal or vertical size of the coding unit and the horizontal orvertical size of the maximum transformation block. For example, when thehorizontal size (width) of the coding unit is larger than the horizontalsize (width) of the maximum transformation block, the coding unit may bevertically bisected. For example, when the vertical size (length) of thecoding unit is larger than the vertical size (length) of the maximumtransformation block, the coding unit may be horizontally bisected.

Information of the maximum and/or minimum size of the coding unit andinformation of the maximum and/or minimum size of the transformationblock may be signaled or determined at an upper level of the codingunit. The upper level may be, for example, a sequence level, a picturelevel, a slice level, a tile group level, a tile level, or the like. Forexample, the minimum size of the coding unit may be determined to be4×4. For example, the maximum size of the transformation block may bedetermined to be 64×64. For example, the minimum size of thetransformation block may be determined to be 4×4.

Information of the minimum size (quad tree minimum size) of a codingunit corresponding to a leaf node of a quad tree and/or information ofthe maximum depth (the maximum tree depth of a multi-type tree) from aroot node to a leaf node of the multi-type tree may be signaled ordetermined at an upper level of the coding unit. For example, the upperlevel may be a sequence level, a picture level, a slice level, a tilegroup level, a tile level, or the like. Information of the minimum sizeof a quad tree and/or information of the maximum depth of a multi-typetree may be signaled or determined for each of an intra-picture sliceand an inter-picture slice.

Difference information between the size of a CTU and the maximum size ofa transformation block may be signaled or determined at an upper levelof the coding unit. For example, the upper level may be a sequencelevel, a picture level, a slice level, a tile group level, a tile level,or the like. Information of the maximum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a maximum size of a binary tree) may be determined basedon the size of the coding tree unit and the difference information. Themaximum size of the coding units corresponding to the respective nodesof a ternary tree (hereinafter, referred to as a maximum size of aternary tree) may vary depending on the type of slice. For example, foran intra-picture slice, the maximum size of a ternary tree may be 32×32.For example, for an inter-picture slice, the maximum size of a ternarytree may be 128×128. For example, the minimum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a minimum size of a binary tree) and/or the minimum sizeof the coding units corresponding to the respective nodes of a ternarytree (hereinafter, referred to as a minimum size of a ternary tree) maybe set as the minimum size of a coding block.

As another example, the maximum size of a binary tree and/or the maximumsize of a ternary tree may be signaled or determined at the slice level.Alternatively, the minimum size of the binary tree and/or the minimumsize of the ternary tree may be signaled or determined at the slicelevel.

Depending on size and depth information of the above-described variousblocks, quad partition information, multi-type tree partition indicationinformation, partition tree information and/or partition directioninformation may be included or may not be included in a bit stream.

For example, when the size of the coding unit is not larger than theminimum size of a quad tree, the coding unit does not contain quadpartition information. Thus, the quad partition information may bededuced from a second value.

For example, when the sizes (horizontal and vertical sizes) of a codingunit corresponding to a node of a multi-type tree are larger than themaximum sizes (horizontal and vertical sizes) of a binary tree and/orthe maximum sizes (horizontal and vertical sizes) of a ternary tree, thecoding unit may not be binary-partitioned or ternary-partitioned.Accordingly, the multi-type tree partition indication information maynot be signaled but may be deduced from a second value.

Alternatively, when the sizes (horizontal and vertical sizes) of acoding unit corresponding to a node of a multi-type tree are the same asthe maximum sizes (horizontal and vertical sizes) of a binary treeand/or are two times as large as the maximum sizes (horizontal andvertical sizes) of a ternary tree, the coding unit may not be furtherbinary-partitioned or ternary-partitioned. Accordingly, the multi-typetree partition indication information may not be signaled but be derivedfrom a second value. This is because when a coding unit is partitionedby a binary tree partition structure and/or a ternary tree partitionstructure, a coding unit smaller than the minimum size of a binary treeand/or the minimum size of a ternary tree is generated.

Alternatively, the binary tree partitioning or the ternary treepartitioning may be limited on the basis of the size of a virtualpipeline data unit (hereinafter, a pipeline buffer size). For example,when the coding unit is divided into sub-coding units which do not fitthe pipeline buffer size by the binary tree partitioning or the ternarytree partitioning, the corresponding binary tree partitioning or ternarytree partitioning may be limited. The pipeline buffer size may be thesize of the maximum transform block (e.g., 64×64). For example, when thepipeline buffer size is 64×64, the division below may be limited.

-   -   N×M (N and/or M is 128) Ternary tree partitioning for coding        units    -   128×N (N<=64) Binary tree partitioning in horizontal direction        for coding units    -   N×128 (N<=64) Binary tree partitioning in vertical direction for        coding units

Alternatively, when the depth of a coding unit corresponding to a nodeof a multi-type tree is equal to the maximum depth of the multi-typetree, the coding unit may not be further binary-partitioned and/orternary-partitioned. Accordingly, the multi-type tree partitionindication information may not be signaled but may be deduced from asecond value.

Alternatively, only when at least one of vertical direction binary treepartitioning, horizontal direction binary tree partitioning, verticaldirection ternary tree partitioning, and horizontal direction ternarytree partitioning is possible for a coding unit corresponding to a nodeof a multi-type tree, the multi-type tree partition indicationinformation may be signaled. Otherwise, the coding unit may not bebinary-partitioned and/or ternary-partitioned. Accordingly, themulti-type tree partition indication information may not be signaled butmay be deduced from a second value.

Alternatively, only when both of the vertical direction binary treepartitioning and the horizontal direction binary tree partitioning orboth of the vertical direction ternary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingunit corresponding to a node of a multi-type tree, the partitiondirection information may be signaled. Otherwise, the partitiondirection information may not be signaled but may be derived from avalue indicating possible partitioning directions.

Alternatively, only when both of the vertical direction binary treepartitioning and the vertical direction ternary tree partitioning orboth of the horizontal direction binary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingtree corresponding to a node of a multi-type tree, the partition treeinformation may be signaled. Otherwise, the partition tree informationmay not be signaled but be deduced from a value indicating a possiblepartitioning tree structure.

FIG. 4 is a view showing an intra-prediction process.

Arrows from center to outside in FIG. 4 may represent predictiondirections of intra prediction modes.

Intra encoding and/or decoding may be performed by using a referencesample of a neighbor block of the current block. A neighbor block may bea reconstructed neighbor block. For example, intra encoding and/ordecoding may be performed by using a coding parameter or a value of areference sample included in a reconstructed neighbor block.

A prediction block may mean a block generated by performing intraprediction. A prediction block may correspond to at least one among CU,PU and TU. A unit of a prediction block may have a size of one among CU,PU and TU. A prediction block may be a square block having a size of2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

Intra prediction may be performed according to intra prediction mode forthe current block. The number of intra prediction modes which thecurrent block may have may be a fixed value and may be a valuedetermined differently according to an attribute of a prediction block.For example, an attribute of a prediction block may comprise a size of aprediction block and a shape of a prediction block, etc.

The number of intra-prediction modes may be fixed to N regardless of ablock size. Or, the number of intra prediction modes may be 3, 5, 9, 17,34, 35, 36, 65, or 67 etc. Alternatively, the number of intra-predictionmodes may vary according to a block size or a color component type orboth. For example, the number of intra prediction modes may varyaccording to whether the color component is a luma signal or a chromasignal. For example, as a block size becomes large, a number ofintra-prediction modes may increase. Alternatively, a number ofintra-prediction modes of a luma component block may be larger than anumber of intra-prediction modes of a chroma component block.

An intra-prediction mode may be a non-angular mode or an angular mode.The non-angular mode may be a DC mode or a planar mode, and the angularmode may be a prediction mode having a specific direction or angle. Theintra-prediction mode may be expressed by at least one of a mode number,a mode value, a mode numeral, a mode angle, and mode direction. A numberof intra-prediction modes may be M, which is larger than 1, includingthe non-angular and the angular mode. In order to intra-predict acurrent block, a step of determining whether or not samples included ina reconstructed neighbor block may be used as reference samples of thecurrent block may be performed. When a sample that is not usable as areference sample of the current block is present, a value obtained byduplicating or performing interpolation on at least one sample valueamong samples included in the reconstructed neighbor block or both maybe used to replace with a non-usable sample value of a sample, thus thereplaced sample value is used as a reference sample of the currentblock.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

As shown in FIG. 7, at least one of the reference sample line 0 to thereference sample line 3 may be used for intra prediction of the currentblock. In FIG. 7, the samples of a segment A and a segment F may bepadded with the samples closest to a segment B and a segment E,respectively, instead of retrieving from the reconstructed neighboringblock. Index information indicating the reference sample line to be usedfor intra prediction of the current block may be signaled. When theupper boundary of the current block is the boundary of the CTU, only thereference sample line 0 may be available. Therefore, in this case, theindex information may not be signaled. When a reference sample lineother than the reference sample line 0 is used, filtering for aprediction block, which will be described later, may not be performed.

When intra-predicting, a filter may be applied to at least one of areference sample and a prediction sample based on an intra-predictionmode and a current block size.

In case of a planar mode, when generating a prediction block of acurrent block, according to a position of a prediction target samplewithin a prediction block, a sample value of the prediction targetsample may be generated by using a weighted sum of an upper and leftside reference sample of a current sample, and a right upper side andleft lower side reference sample of the current block. In addition, incase of a DC mode, when generating a prediction block of a currentblock, an average value of upper side and left side reference samples ofthe current block may be used. In addition, in case of an angular mode,a prediction block may be generated by using an upper side, a left side,a right upper side, and/or a left lower side reference sample of thecurrent block. In order to generate a prediction sample value,interpolation of a real number unit may be performed.

In the case of intra prediction between color components, a predictionblock for the current block of the second color component may begenerated on the basis of the corresponding reconstructed block of thefirst color component. For example, the first color component may be aluma component, and the second color component may be a chromacomponent. For intra prediction between color components, the parametersof the linear model between the first color component and the secondcolor component may be derived on the basis of the template. Thetemplate may include upper and/or left neighboring samples of thecurrent block and upper and/or left neighboring samples of thereconstructed block of the first color component corresponding thereto.For example, the parameters of the linear model may be derived using asample value of a first color component having a maximum value amongsamples in a template and a sample value of a second color componentcorresponding thereto, and a sample value of a first color componenthaving a minimum value among samples in the template and a sample valueof a second color component corresponding thereto. When the parametersof the linear model are derived, a corresponding reconstructed block maybe applied to the linear model to generate a prediction block for thecurrent block. According to a video format, subsampling may be performedon the neighboring samples of the reconstructed block of the first colorcomponent and the corresponding reconstructed block. For example, whenone sample of the second color component corresponds to four samples ofthe first color component, four samples of the first color component maybe sub-sampled to compute one corresponding sample. In this case, theparameter derivation of the linear model and intra prediction betweencolor components may be performed on the basis of the correspondingsub-sampled samples. Whether or not to perform intra prediction betweencolor components and/or the range of the template may be signaled as theintra prediction mode.

The current block may be partitioned into two or four sub-blocks in thehorizontal or vertical direction. The partitioned sub-blocks may besequentially reconstructed. That is, the intra prediction may beperformed on the sub-block to generate the sub-prediction block. Inaddition, dequantization and/or inverse transform may be performed onthe sub-blocks to generate sub-residual blocks. A reconstructedsub-block may be generated by adding the sub-prediction block to thesub-residual block. The reconstructed sub-block may be used as areference sample for intra prediction of the sub-sub-blocks. Thesub-block may be a block including a predetermined number (for example,16) or more samples. Accordingly, for example, when the current block isan 8×4 block or a 4×8 block, the current block may be partitioned intotwo sub-blocks. Also, when the current block is a 4×4 block, the currentblock may not be partitioned into sub-blocks. When the current block hasother sizes, the current block may be partitioned into four sub-blocks.Information on whether or not to perform the intra prediction based onthe sub-blocks and/or the partitioning direction (horizontal orvertical) may be signaled. The intra prediction based on the sub-blocksmay be limited to be performed only when reference sample line 0 isused. When the intra prediction based on the sub-block is performed,filtering for the prediction block, which will be described later, maynot be performed.

The final prediction block may be generated by performing filtering onthe prediction block that is intra-predicted. The filtering may beperformed by applying predetermined weights to the filtering targetsample, the left reference sample, the upper reference sample, and/orthe upper left reference sample. The weight and/or the reference sample(range, position, etc.) used for the filtering may be determined on thebasis of at least one of a block size, an intra prediction mode, and aposition of the filtering target sample in the prediction block. Thefiltering may be performed only in the case of a predetermined intraprediction mode (e.g., DC, planar, vertical, horizontal, diagonal,and/or adjacent diagonal modes). The adjacent diagonal mode may be amode in which k is added to or subtracted from the diagonal mode. Forexample, k may be a positive integer of 8 or less.

An intra-prediction mode of a current block may be entropyencoded/decoded by predicting an intra-prediction mode of a blockpresent adjacent to the current block. When intra-prediction modes ofthe current block and the neighbor block are identical, information thatthe intra-prediction modes of the current block and the neighbor blockare identical may be signaled by using predetermined flag information.In addition, indicator information of an intra-prediction mode that isidentical to the intra-prediction mode of the current block amongintra-prediction modes of a plurality of neighbor blocks may besignaled. When intra-prediction modes of the current block and theneighbor block are different, intra-prediction mode information of thecurrent block may be entropy encoded/decoded by performing entropyencoding/decoding based on the intra-prediction mode of the neighborblock.

FIG. 5 is a diagram illustrating an embodiment of an inter-pictureprediction process.

In FIG. 5, a rectangle may represent a picture. In FIG. 5, an arrowrepresents a prediction direction. Pictures may be categorized intointra pictures (I pictures), predictive pictures (P pictures), andBi-predictive pictures (B pictures) according to the encoding typethereof.

The I picture may be encoded through intra-prediction without requiringinter-picture prediction. The P picture may be encoded throughinter-picture prediction by using a reference picture that is present inone direction (i.e., forward direction or backward direction) withrespect to a current block. The B picture may be encoded throughinter-picture prediction by using reference pictures that are preset intwo directions (i.e., forward direction and backward direction) withrespect to a current block. When the inter-picture prediction is used,the encoder may perform inter-picture prediction or motion compensationand the decoder may perform the corresponding motion compensation.

Hereinbelow, an embodiment of the inter-picture prediction will bedescribed in detail.

The inter-picture prediction or motion compensation may be performedusing a reference picture and motion information.

Motion information of a current block may be derived duringinter-picture prediction by each of the encoding apparatus 100 and thedecoding apparatus 200. The motion information of the current block maybe derived by using motion information of a reconstructed neighboringblock, motion information of a collocated block (also referred to as acol block or a co-located block), and/or a block adjacent to theco-located block. The co-located block may mean a block that is locatedspatially at the same position as the current block, within a previouslyreconstructed collocated picture (also referred to as a col picture or aco-located picture). The co-located picture may be one picture among oneor more reference pictures included in a reference picture list.

The derivation method of the motion information may be differentdepending on the prediction mode of the current block. For example, aprediction mode applied for inter prediction includes an AMVP mode, amerge mode, a skip mode, a merge mode with a motion vector difference, asubblock merge mode, a triangle partition mode, an inter-intracombination prediction mode, affine mode, and the like. Herein, themerge mode may be referred to as a motion merge mode.

For example, when the AMVP is used as the prediction mode, at least oneof motion vectors of the reconstructed neighboring blocks, motionvectors of the co-located blocks, motion vectors of blocks adjacent tothe co-located blocks, and a (0, 0) motion vector may be determined asmotion vector candidates for the current block, and a motion vectorcandidate list is generated by using the emotion vector candidates. Themotion vector candidate of the current block can be derived by using thegenerated motion vector candidate list. The motion information of thecurrent block may be determined based on the derived motion vectorcandidate. The motion vectors of the collocated blocks or the motionvectors of the blocks adjacent to the collocated blocks may be referredto as temporal motion vector candidates, and the motion vectors of thereconstructed neighboring blocks may be referred to as spatial motionvector candidates.

The encoding apparatus 100 may calculate a motion vector difference(MVD) between the motion vector of the current block and the motionvector candidate and may perform entropy encoding on the motion vectordifference (MVD). In addition, the encoding apparatus 100 may performentropy encoding on a motion vector candidate index and generate abitstream. The motion vector candidate index may indicate an optimummotion vector candidate among the motion vector candidates included inthe motion vector candidate list. The decoding apparatus may performentropy decoding on the motion vector candidate index included in thebitstream and may select a motion vector candidate of a decoding targetblock from among the motion vector candidates included in the motionvector candidate list by using the entropy-decoded motion vectorcandidate index. In addition, the decoding apparatus 200 may add theentropy-decoded MVD and the motion vector candidate extracted throughthe entropy decoding, thereby deriving the motion vector of the decodingtarget block.

Meanwhile, the coding apparatus 100 may perform entropy-coding onresolution information of the calculated MVD. The decoding apparatus 200may adjust the resolution of the entropy-decoded MVD using the MVDresolution information.

Meanwhile, the coding apparatus 100 calculates a motion vectordifference (MVD) between a motion vector and a motion vector candidatein the current block on the basis of an affine model, and performsentropy-coding on the MVD. The decoding apparatus 200 derives a motionvector on a per sub-block basis by deriving an affine control motionvector of a decoding target block through the sum of the entropy-decodedMVD and an affine control motion vector candidate.

The bitstream may include a reference picture index indicating areference picture. The reference picture index may be entropy-encoded bythe encoding apparatus 100 and then signaled as a bitstream to thedecoding apparatus 200. The decoding apparatus 200 may generate aprediction block of the decoding target block based on the derivedmotion vector and the reference picture index information.

Another example of the method of deriving the motion information of thecurrent may be the merge mode. The merge mode may mean a method ofmerging motion of a plurality of blocks. The merge mode may mean a modeof deriving the motion information of the current block from the motioninformation of the neighboring blocks. When the merge mode is applied,the merge candidate list may be generated using the motion informationof the reconstructed neighboring blocks and/or the motion information ofthe collocated blocks. The motion information may include at least oneof a motion vector, a reference picture index, and an inter-pictureprediction indicator. The prediction indicator may indicateone-direction prediction (L0 prediction or L1 prediction) ortwo-direction predictions (L0 prediction and L1 prediction).

The merge candidate list may be a list of motion information stored. Themotion information included in the merge candidate list may be at leastone of motion information (spatial merge candidate) of a neighboringblock adjacent to the current block, motion information (temporal mergecandidate) of the collocated block of the current block in the referencepicture, new motion information generated by a combination of the motioninformation exiting in the merge candidate list, motion information(history-based merge candidate) of the block that is encoded/decodedbefore the current block, and zero merge candidate.

The encoding apparatus 100 may generate a bitstream by performingentropy encoding on at least one of a merge flag and a merge index andmay signal the bitstream to the decoding apparatus 200. The merge flagmay be information indicating whether or not to perform the merge modefor each block, and the merge index may be information indicating thatwhich neighboring block, among the neighboring blocks of the currentblock, is a merge target block. For example, the neighboring blocks ofthe current block may include a left neighboring block on the left sideof the current block, an upper neighboring block disposed above thecurrent block, and a temporal neighboring block temporally adjacent tothe current block.

Meanwhile, the coding apparatus 100 performs entropy-coding on thecorrection information for correcting the motion vector among the motioninformation of the merge candidate and signals the same to the decodingapparatus 200. The decoding apparatus 200 can correct the motion vectorof the merge candidate selected by the merge index on the basis of thecorrection information. Here, the correction information may include atleast one of information on whether or not to perform the correction,correction direction information, and correction size information. Asdescribed above, the prediction mode that corrects the motion vector ofthe merge candidate on the basis of the signaled correction informationmay be referred to as a merge mode having the motion vector difference.

The skip mode may be a mode in which the motion information of theneighboring block is applied to the current block as it is. When theskip mode is applied, the encoding apparatus 100 may perform entropyencoding on information of the fact that the motion information of whichblock is to be used as the motion information of the current block togenerate a bit stream, and may signal the bitstream to the decodingapparatus 200. The encoding apparatus 100 may not signal a syntaxelement regarding at least any one of the motion vector differenceinformation, the encoding block flag, and the transform coefficientlevel to the decoding apparatus 200.

The subblock merge mode may mean a mode that derives the motioninformation in units of sub-blocks of a coding block (CU). When thesubblock merge mode is applied, a subblock merge candidate list may begenerated using motion information (sub-block based temporal mergecandidate) of the sub-block collocated to the current sub-block in thereference image and/or an affine control point motion vector mergecandidate.

The triangle partition mode may mean a mode that derives motioninformation by partitioning the current block into diagonal directions,derives each prediction sample using each of the derived motioninformation, and derives the prediction sample of the current block byweighting each of the derived prediction samples.

The inter-intra combined prediction mode may mean a mode that derives aprediction sample of the current block by weighting a prediction samplegenerated by inter prediction and a prediction sample generated by intraprediction.

The decoding apparatus 200 may correct the derived motion information byitself. The decoding apparatus 200 may search the predetermined regionon the basis of the reference block indicated by the derived motioninformation and derive the motion information having the minimum SAD asthe corrected motion information.

The decoding apparatus 200 may compensate a prediction sample derivedvia inter prediction using an optical flow.

FIG. 6 is a diagram illustrating a transform and quantization process.

As illustrated in FIG. 6, a transform and/or quantization process isperformed on a residual signal to generate a quantized level signal. Theresidual signal is a difference between an original block and aprediction block (i.e., an intra prediction block or an inter predictionblock). The prediction block is a block generated through intraprediction or inter prediction. The transform may be a primarytransform, a secondary transform, or both. The primary transform of theresidual signal results in transform coefficients, and the secondarytransform of the transform coefficients results in secondary transformcoefficients.

At least one scheme selected from among various transform schemes whichare preliminarily defined is used to perform the primary transform. Forexample, examples of the predefined transform schemes include discretecosine transform (DCT), discrete sine transform (DST), andKarhunen-Loeve transform (KLT). The transform coefficients generatedthrough the primary transform may undergo the secondary transform. Thetransform schemes used for the primary transform and/or the secondarytransform may be determined according to coding parameters of thecurrent block and/or neighboring blocks of the current block.Alternatively, transform information indicating the transform scheme maybe signaled. The DCT-based transform may include, for example, DCT-2,DCT-8, and the like. The DST-based transform may include, for example,DST-7.

A quantized-level signal (quantization coefficients) may be generated byperforming quantization on the residual signal or a result of performingthe primary transform and/or the secondary transform. The quantizedlevel signal may be scanned according to at least one of a diagonalup-right scan, a vertical scan, and a horizontal scan, depending on anintra prediction mode of a block or a block size/shape. For example, asthe coefficients are scanned in a diagonal up-right scan, thecoefficients in a block form change into a one-dimensional vector form.Aside from the diagonal up-right scan, the horizontal scan ofhorizontally scanning a two-dimensional block form of coefficients orthe vertical scan of vertically scanning a two-dimensional block form ofcoefficients may be used depending on the intra prediction mode and/orthe size of a transform block. The scanned quantized-level coefficientsmay be entropy-encoded to be inserted into a bitstream.

A decoder entropy-decodes the bitstream to obtain the quantized-levelcoefficients. The quantized-level coefficients may be arranged in atwo-dimensional block form through inverse scanning. For the inversescanning, at least one of a diagonal up-right scan, a vertical scan, anda horizontal scan may be used.

The quantized-level coefficients may then be dequantized, then besecondary-inverse-transformed as necessary, and finally beprimary-inverse-transformed as necessary to generate a reconstructedresidual signal.

Inverse mapping in a dynamic range may be performed for a luma componentreconstructed through intra prediction or inter prediction beforein-loop filtering. The dynamic range may be divided into 16 equal piecesand the mapping function for each piece may be signaled. The mappingfunction may be signaled at a slice level or a tile group level. Aninverse mapping function for performing the inverse mapping may bederived on the basis of the mapping function. In-loop filtering,reference picture storage, and motion compensation are performed in aninverse mapped region, and a prediction block generated through interprediction is converted into a mapped region via mapping using themapping function, and then used for generating the reconstructed block.However, since the intra prediction is performed in the mapped region,the prediction block generated via the intra prediction may be used forgenerating the reconstructed block without mapping/inverse mapping.

When the current block is a residual block of a chroma component, theresidual block may be converted into an inverse mapped region byperforming scaling on the chroma component of the mapped region. Theavailability of the scaling may be signaled at the slice level or thetile group level. The scaling may be applied only when the mapping forthe luma component is available and the division of the luma componentand the division of the chroma component follow the same tree structure.The scaling may be performed on the basis of an average of sample valuesof a luma prediction block corresponding to the color difference block.In this case, when the current block uses inter prediction, the lumaprediction block may mean a mapped luma prediction block. A valuenecessary for the scaling may be derived by referring to a lookup tableusing an index of a piece to which an average of sample values of a lumaprediction block belongs. Finally, by scaling the residual block usingthe derived value, the residual block may be switched to the inversemapped region. Then, chroma component block restoration, intraprediction, inter prediction, in-loop filtering, and reference picturestorage may be performed in the inverse mapped area.

Information indicating whether the mapping/inverse mapping of the lumacomponent and chroma component is available may be signaled through aset of sequence parameters.

The prediction block of the current block may be generated on the basisof a block vector indicating a displacement between the current blockand the reference block in the current picture. In this way, aprediction mode for generating a prediction block with reference to thecurrent picture is referred to as an intra block copy (IBC) mode. TheIBC mode may be applied to M×N (M<=64, N<=64) coding units. The IBC modemay include a skip mode, a merge mode, an AMVP mode, and the like. Inthe case of a skip mode or a merge mode, a merge candidate list isconstructed, and the merge index is signaled so that one merge candidatemay be specified. The block vector of the specified merge candidate maybe used as a block vector of the current block. The merge candidate listmay include at least one of a spatial candidate, a history-basedcandidate, a candidate based on an average of two candidates, and azero-merge candidate. In the case of an AMVP mode, the difference blockvector may be signaled. In addition, the prediction block vector may bederived from the left neighboring block and the upper neighboring blockof the current block. The index on which neighboring block to use may besignaled. The prediction block in the IBC mode is included in thecurrent CTU or the left CTU and limited to a block in the alreadyreconstructed area. For example, a value of the block vector may belimited such that the prediction block of the current block ispositioned in an area of three 64×64 blocks preceding the 64×64 block towhich the current block belongs in the coding/decoding order. Bylimiting the value of the block vector in this way, memory consumptionand device complexity according to the IBC mode implementation may bereduced.

In a general entropy encoder or decoder, a probability value used inencoding or decoding of binary symbols (bin) may be dependent on apreviously encoded or decoded binary value and probability value.Therefore, in order to perform encoding or decoding of the current bin,there is a problem that entropy encoding or decoding of the previous binand a context update need to be completed. Because of this problem,throughput may increase in general entropy coding. In thisspecification, the throughput may refer to the number of bins to beencoded or decoded per unit time.

In the present invention, in order to solve this problem, dependency onthe probability value used in entropy encoding or decoding of theprevious bin and the current bin on a per-any-block basis may beremoved, or the same syntax elements may be grouped. As a result of theremoval of the dependency and the grouping, the throughput of theencoder or the decoder may be enhanced. Further, according to thepresent invention, one or more probability values may be updatedsimultaneously using pieces of previous binary information and aprobability distribution model, and thus throughput of the encoder orthe decoder may be enhanced.

Hereinafter, according to an embodiment of the present invention, amethod of encoding or decoding an image by using entropy coding will bedescribed in detail. Herein, the entropy coding may refer to entropyencoding in terms of an encoder and may refer to entropy decoding interms of a decoder.

FIG. 8 is a diagram illustrating an entropy decoder according to anembodiment of the present invention.

According to FIG. 8, a bitstream may be input to an entropy decodingmodule and a context update module. Herein, the entropy decoding modulemay refer to an arithmetic entropy decoding module or a binaryarithmetic entropy decoding module. Further, in an embodiment of thepresent invention, entropy encoding/decoding may refer to arithmeticentropy encoding/decoding.

The entropy decoding module and the context update module may performentropy decoding and the context update using a context corresponding toa syntax element to be currently decoded. In this specification, thecontext may refer to occurrence probability information for each bin ora binary value of an already encoded or decoded syntax element. Thecontext update module may perform the context update to apply thecurrently decoded probability information to entropy decoding of theimmediately subsequent bin, and may store the updated context in acontext memory. Herein, the context corresponding to the syntax elementto be currently decoded may be derived by a context modeler. Further,the context corresponding to the bin within the syntax element to becurrently decoded may be derived by the context modeler. For example,the context modeler may consist of a context selection module and acontext memory. The context corresponding to the current bin selected bythe context selection module may be loaded from the context memory to beused for entropy decoding. After entropy decoding is performed, adebinarization module may perform a debinarization process in which atleast one of binary values is transformed into a syntax element form.The debinarization module may perform debinarization on at least one ofthe input binary values and may output syntax element information.Herein, at the same time as the debinarization process, the currentsyntax element and binary information may be transmitted to the contextmodeler, and may be used to select the context for the subsequent bin tobe decoded.

In the decoder according to FIG. 8, the probability information used indecoding of the current bin is dependent on the previously decodedbinary value. Therefore, only after entropy decoding for the previousbin and the probability update are completed, entropy decoding of thecurrent bin starts. This may cause a problem of increase in throughput.

FIG. 9 is a diagram illustrating an entropy decoder according to anotherembodiment of the present invention.

The decoder according to the present invention may not perform thecontext update within any predefined unit (hereinafter, referred to asan entropy coding unit (ECU)) in order to enhance throughput by removingprobability dependency on the previous bin and the current bin. That is,the entropy decoder may perform the context update of the syntax elementon a per-ECU basis.

For example, in this specification, the ECU may refer to any blockhaving an N×M size. Herein, N and M may be positive integers.

As another example, the ECU may be at least one among a picture, a CTU,a subpicture, a tile, a brick, a slice, a CU, a PU, a TU, a CTB, a CB, aPB, and a TB. As still another example, the ECU may refer to a subblockconstituting at least one among a picture, a CTU, a subpicture, a tile,a brick, a slice, a CU, a PU, a TU, a CTB, a CB, a PB, and a TB. Herein,the subblock may refer to a basic unit in which independent transformcoefficient scanning is performed in the entropy encoding or decodingprocess.

As still another example, the ECU may refer to a unit including apredefined number of bins. Herein, the predefined number of bins may bea positive number N other than 0. That is, the ECU may be changed on thebasis of N bins. Herein, the number N of bins may refer to the number ofcontext bins in which the context update is performed except for bypassbins in which the context update is not performed. For example, N may bea fixed value that is transmitted through at least one among a sequence,a picture, a tile, a slice (segment) header, and a parameter set, orthat is used in the encoder and the decoder. As still another example, Nmay be determined on the basis of at least one of coding parameterstransmitted from the encoder to the decoder. As still another example, Nmay be determined on the basis of at least one of coding parameters ofthe current block. For example, N may be determined on the basis of atleast one among the size, the depth, the shape, the prediction mode, andthe transform mode of the current block including the current bin.Herein, the size of the block may include at least one among thehorizontal size of the block and the vertical size of the block.

For example, the decoder in FIG. 9 may be an example of the improvementof the decoder in FIG. 8. Compared to the entropy decoder in FIG. 8, theentropy decoder in FIG. 9 may not include the context update module.

According to FIG. 9, the entropy decoding module may load the contextfor the current bin from the context memory and may perform entropydecoding, without the context update. Herein, the update on the contextmay not be performed within the ECU. According to the present invention,waiting for the context update for the previous bin or a memory accessoperation for loading the context is not performed, so that throughputof entropy decoding may be enhanced.

FIGS. 10A-10B are diagrams illustrating an entropy encoding method andan entropy decoding method according to an embodiment of the presentinvention.

The entropy encoding method according to the embodiment of the presentinvention may include generating a bin for a syntax element of a currentblock by binarizing the syntax element at step S1010 a, updating acontext used in entropy encoding of the syntax element of the currentblock at step S1020 a, and generating a bitstream for the syntax elementof the current block by using the generated bin and the updated contextat step S1030 a. Herein, the context update may be performed on aper-ECU basis. Further, when the syntax element of the current block isalready binarized, the step of generating of the bin for the syntaxelement is omitted, and the bin in the binarized syntax element issubjected to entropy encoding and then generated as a bitstream.Further, the bitstream for the syntax element of the current block maybe generated using only the bin. For example, according to the type orvalue of the syntax element of the current block, whether to performbinarization may be determined.

The entropy decoding method according to the embodiment of the presentinvention may include updating a context used in entropy decoding of asyntax element of a current block at step S1010 b, generating a bin forthe syntax element of the current block by entropy decoding a bitstreamon the basis of the updated context at step S1020 b, and obtaining thesyntax element of the current block by debinarizing the generated bin atstep S1030 b. Herein, the context update may be performed on a per-ECUbasis. Further, when the syntax element of the current block is alreadybinarized, the process of debinarization is omitted, and the syntaxelement of the current block is obtained using at least one of thegenerated bins. For example, according to the type or value of thesyntax element of the current block, whether to perform debinarizationmay be determined.

The syntax element may include at least one bin.

FIGS. 11A-11B are diagrams illustrating an entropy encoding method andan entropy decoding method according to another embodiment of thepresent invention.

The entropy encoding method according to the embodiment of the presentinvention may include determining whether to perform a context updatefor a first syntax element of a current block at step S1110 a, updatinga context for entropy encoding of the first syntax element on the basisof the determination at step S1120 a, and generating a bitstream for thecurrent block by using the updated context and the generated bin at stepS1130 a. Further, the bitstream for the syntax element of the currentblock may be generated using only the bin.

Herein, whether to perform the context update may be determined on thebasis of the number of pre-encoded predetermined syntax elements or binsfor the current block.

The entropy decoding method according to the embodiment of the presentinvention may include determining whether to perform a context updatefor a first syntax element of a current block at step S1110 b, updatinga context for entropy decoding of the first syntax element on the basisof the determination at step S1120 b, and generating a bin for the firstsyntax element on the basis of the updated context at step S1130 b.Further, the bin for the first syntax element may be generated withoutusing the updated context.

Herein, whether to perform the context update may be determined on thebasis of the number of pre-decoded predetermined syntax elements or binsfor the current block.

FIG. 12 is another diagram illustrating an entropy encoding methodaccording to an embodiment of the present invention.

The entropy encoding method according to the present invention will bedescribed in detail with reference to FIG. 12. The encoder according tothe present invention may perform the context update on a per-ECU basis.For the first ECU of a picture or tile, the encoder may perform entropyencoding after initializing the context with a predefined probabilityvalue.

The embodiment in FIG. 12 may be an embodiment of a case in which it isassumed that an ECU and a CTU are the same in size. Therefore, theencoder may perform the context update on a per-CTU basis. The encodermay determine that entropy encoding of the current CTU is completed, byusing a syntax element, such as a syntax element end_of_tile_flag, orthe like, for identifying the CTU boundary. That is, when the syntaxelement end_of_tile_flag, or the like is encoded, the encoder determinesthat the subsequent syntax element is a syntax element included in thesubsequent ECU, performs an update on all contexts, and performs entropyencoding. Herein, during entropy encoding for one ECU, the context maynot be additionally updated.

For example, when the size of the ECU is equal to or smaller than thesize of the CTU, the encoder calculates the area of the encoded CU anddetermines ECU boundaries for syntax elements.

For example, in the case where the ECU is a size of 32×32, when encodingof the CU corresponding to a 32×32 area is completed, the encoderdetermines that the subsequent syntax elements are included in thesubsequent ECU.

As another example, when the size of the ECU is defined as the maximumtransform size, the encoder calculates the area of an encoded TU anddetermines the ECU boundaries for syntax elements.

For example, in the case where the ECU is defined as having the maximumtransform size 64×64, when encoding of the TU corresponding to a 64×64area is completed, the encoder determines that the subsequent syntaxelements are included in the subsequent ECU.

The above-described area is an example, and the area of the ECU may bedefined as N×M. Herein, N and M may be positive integers.

As still another example, in the case where the ECU refers to a subblockthat is subjected to independent transform coefficient scanning in theentropy encoding process, when the encoder determines that an index forthe subblock is changed during the entropy encoding process for theresidual signal-related syntax element included in the subblock, theencoder determines that the ECU is changed.

As still another example, in the case where the ECU refers to a subblockthat is subjected to independent transform coefficient scanning in theentropy encoding process, the encoder may determine the ECU boundariesusing the number of encoded transform coefficients.

As still another example, in the case where the ECU is a unit includinga predefined number of bins, the encoder may encode the preset N binsand then may determine that the ECU is changed.

FIG. 13 is another diagram illustrating an entropy decoding methodaccording to an embodiment of the present invention.

The entropy decoding method according to the present invention will bedescribed in detail with reference to FIG. 13. The decoder according tothe present invention may perform the context update on a per-ECU basis.For the first ECU of the picture of the tile, the decoder may performentropy decoding after initializing the context with a predefinedprobability value.

The embodiment in FIG. 13 may be an embodiment of a case in which it isassumed that an ECU and a CTU are the same in size. Therefore, thedecoder may perform the context update on a per-CTU basis. The decodermay determine that entropy decoding of the current CTU is completed, byusing a syntax element, such as a syntax element end_of_tile_flag, orthe like, for identifying the CTU boundary. That is, when the syntaxelement end_of_tile_flag, or the like is decoded, the decoder determinesthat the subsequent syntax element is a syntax element included in thesubsequent ECU, performs an update on all contexts, and performs entropydecoding. Herein, during entropy decoding for one ECU, the context maynot be additionally updated.

For example, when the size of the ECU is equal to or smaller than thesize of the CTU, the decoder calculates the area of the decoded CU anddetermines the ECU boundary for syntax elements.

For example, in the case were the ECU is a size of 32×32, when decodingof the CU corresponding to an 32×32 area is completed, the decoderdetermines that the subsequent syntax elements are included in thesubsequent ECU.

As another example, when the size of the ECU is defined as the maximumtransform size, the decoder calculates the area of a decoded TU anddetermines the ECU boundary for syntax elements.

For example, in the case where the ECU is defined as having the maximumtransform size 64×64, when decoding of the TU corresponding to a 64×64area is completed, the decoder determines that the subsequent syntaxelements are included in the subsequent ECU.

The above-described area is an example, and the area of the ECU may bedefined as N×M. Herein, N and M may be positive integers.

As still another example, in the case where the ECU refers to a subblockthat is subjected to independent transform coefficient scanning in theentropy decoding process, when the decoder determines that an index forthe subblock is changed during the entropy decoding process for theresidual signal-related syntax element included in the subblock, thedecoder determines that the ECU is changed.

As still another example, in the case where the ECU refers to a subblockthat is subjected to independent transform coefficient scanning in theentropy decoding process, the decoder may determine the ECU boundaryusing the number of decoded transform coefficient.

For example, in the case where the size of the subblock is predefined as4×4, 2×8, 8×2, or the like, when decoding of 16 transformcoefficient-related syntax elements is completed in the scanning order,the decoder determines that the subsequent residual signal-relatedsyntax elements are included in the subsequent subblock (namely, theECU). Herein, the number of transform coefficients may be calculatedfrom residual signal-related syntax elements that are explicitlytransmitted as well as syntax elements for a residual signal which maybe implicitly derived from at least one syntax element of syntaxelements last_sig_coeff_x_prefix, last_sig_coeff_y_prefix,last_sig_coeff_x_suffix, last_sig_coeff_y_suffix, coded_sub_block_flag,sig_coeff_flag, par_level_flag, abs_level_gtx_flag, abs_remainder,dec_abs_level, coeff_sign_flag, and the like.

Herein, at least one among last_sig_coeff_x_prefix,last_sig_coeff_y_prefix, last_sig_coeff_x_suffix,last_sig_coeff_y_suffix, coded_sub_block_flag, sig_coeff_flag,par_level_flag, abs_level_gtx_flag, abs_remainder, dec_abs_level, andcoeff_sign_flag may refer to a syntax element used to entropyencode/decode the residual signal of the current block.

As still another example, in the case where the ECU is a unit includinga predefined number of bins, the decoder may decode the preset N binsand then may determine that the ECU is changed.

According to another embodiment of the present invention, the encoder orthe decoder may perform the context update only for the bins included inthe ECU and may not perform the context update for the bins not includedin the ECU.

For example, in the case where the ECU is a unit including a predefinednumber of syntax elements or bins, the encoder or decoder may performthe context update only for the predefined number of bins during syntaxelement encoding or decoding on the current block and may not performthe context update for the bins exceeding the predefined number.

For example, when the encoder or decoder entropy encodes or decodes atleast one of any syntax elements for the current block, the encoder ordecoder performs the context update only for at least one of thepredefined number of the syntax elements and does not perform thecontext update for at least one of the remaining syntax elements.

Herein, the bin subjected to the context update may be referred to as acontext bin, and a process of encoding/decoding the context bin may bereferred to as context coding. Further, the bin not subjected to thecontext update may be referred to as a bypass bin, and a process ofencoding/decoding the bypass bin may be referred to as bypass coding.

As another example, the encoder or decoder may calculate the number oftimes that at least one of the predetermined syntax elements in thecurrent block is encoded or decoded (parsed), in order to determinewhether to perform the context update for the first syntax element ofthe current block. For example, when the number of times that at leastone of the predetermined syntax elements in the current block is encodedor decoded exceeds a predefined value, the encoder or decoder does notperform the context update for the first syntax element. However, whenthe number of times that at least one of the predetermined syntaxelements in the current block is parsed is equal to or less than thepredefined value, the encoder or decoder performs the context update forthe first syntax element.

In order to calculate the number of times that the first syntax elementis parsed, the encoder or decoder may set a variable for calculating thenumber of times that at least one of the predetermined syntax elementsis encoded or decoded. Herein, an initial value of the variable forcalculating the number of times that the at least one of thepredetermined syntax elements is encoded or decoded may be determined bythe number N of bins predefined by the ECU. As another example, aninitial value of the variable for calculating the number of times thatthe at least one of the predetermined syntax elements is encoded ordecoded may be determined on the basis of a coding parameter of thecurrent block. For example, the initial value of the variable forcalculating the number of times that the syntax element is parsed may bedetermined on the basis of at least one among the size, the depth, andthe shape of the current block. Herein, the size of the block mayinclude at least one among the horizontal size of the block and thevertical size of the block.

Each time at least one of the predetermined syntax elements is encodedor decoded, the encoder or decoder reduces the value of the variable forcalculating the number of times that the syntax element is encoded ordecoded, and thus determines the number of encoded or decoded syntaxelements. That is, when the variable becomes equal or less than aparticular value, the encoder or decoder determines that the predefinednumber of the syntax elements are encoded or decoded. When a predefinednumber of context updates are performed for at least one ofpredetermined syntax elements, the encoder or decoder performs entropyencoding or decoding on the first syntax element to be encoded ordecoded later without the context update.

For example, at least one of predetermined syntax elements may be asyntax element accompanied by the context update. That is, even thoughencoding or decoding is performed for any syntax element, when thesyntax element is a syntax element that is not accompanied by thecontext update, encoding or decoding of the syntax element is notincluded in the above-described number of times that encoding ordecoding is performed. That is, when any syntax element is the syntaxelement that is not accompanied by the context update, the encoder ordecoder does not reduce the variable for calculating the number of timesthat the predetermined syntax element is encoded or decoded.

For example, the syntax element that is not accompanied by the contextupdate may be coded_sub_block_flag, abs_remainder, or the like.

For example, in the present invention, the first syntax element may becoeff_sign_flag. Further, the predetermined syntax element may be atleast one among coeff_sign_flag, sig_coeff_flag, abs_level_gtx_flag, andpar_level_flag.

Herein, the syntax element coeff_sign_flag may indicate a sign of alevel of a transform coefficient. The syntax element sig_coeff_flag mayindicate whether a quantized level at a particular position is 0. Thesyntax element abs_level_gtx_flag may be related to an absolute value ofa quantized level at a particular position. The syntax elementpar_level_flag may indicate parity of a quantized level at a particularposition.

Further, the syntax element coded_sub_block_flag may indicate whether aquantization level encoded on a per-subblock basis is present. Thesyntax element abs_remainder may be related to a residual size value ofan absolute value of a quantized level.

As still another example, the above embodiment in which whether toperform the context update for the syntax element to be encoded ordecoded is determined may be performed only when the current block isencoded or decoded in a transform skip mode. That is, when the currentblock is not in the transform skip mode, the encoder or decoder does notperform the context update for the syntax element to be encoded ordecoded.

Hereinafter, the above-described binarization or debinarization stepwill be described in detail.

The encoder may perform a binarization process in which syntax elementsto be encoded are transformed into a binary string, before performingentropy encoding. The binarized binary string may be used for thecontext update and entropy encoding in order, starting from the firstbinary value transformed.

Regarding binarization for each of the syntax elements, at least oneamong an optimum method considering the probability of occurrence ofeach value within a range of values that each of the syntax elements mayhave, and a fixed method predefined by the encoder may be used. Forexample, at least one of the following methods may be selectively usedaccording to each syntax element: truncated rice binarization, K-thorder exponential Golomb binarization, limited K-th order exponentialGolomb binarization, fixed-length binarization, unary binarization, andtruncated unary or truncated binary binarization methods.

For example, when the probability distribution of values of the MVDsyntax element, which represents the absolute value of the difference ofmotion vectors, is similar to Laplacian probability distribution, theencoder performs binarization using unary binarization.

As another example, the encoder may change the binarization method usingstatistical information of the syntax element previously encoded foreach syntax element.

For example, from the first block to the current block of the slice,tile, or picture, binarization is performed using a predefined 0-thorder exponential Golomb binarization method. However, when theprobability distribution of the encoded MVD syntax element valuesindicates probability distribution appropriate to the 1-th orderexponential Golomb, the encoder performs binarization for the MVD syntaxelement by using 1-th order exponential Golomb binarization.

In the meantime, the decoder may perform a debinarization process inwhich the value of the syntax element is output using a binary stringoutput through entropy decoding. The output binary string may be usedfor context update and decoding processes.

Regarding debinarization for each of the syntax elements, at least oneamong an optimum method considering the probability of occurrence ofeach value within a range of values that each of the syntax elements mayhave, and a fixed method predefined by the decoder may be used. Forexample, at least one of the following methods may be selectively usedaccording to each syntax element: truncated rice debinarization, K-thorder exponential Golomb debinarization, limited K-th order exponentialGolomb debinarization, fixed-length binarization, unary debinarization,and truncated unary or truncated binary debinarization methods.

For example, when the probability distribution of values of the MVDsyntax element, which represents the absolute value of the difference ofmotion vectors, is similar to Laplacian probability distribution, thedecoder performs debinarization using unary debinarization.

As another example, the decoder may change the debinarization methodusing statistical information of the syntax element previously decodedfor each syntax element.

For example, from the first block to the current block of the slice,tile, or picture, debinarization is performed using a predefined 0-thorder exponential Golomb binarization method. However, when theprobability distribution of the decoded MVD syntax element valuesindicates probability distribution appropriate to the 1-th orderexponential Golomb, the decoder performs debinarization for the MVDsyntax element by using 1-th order exponential Golomb debinarization.

Hereinafter, the above-described context update step of the encoder ordecoder will be described in detail.

According to an embodiment of the present invention, the encoder ordecoder may update the context on a per-ECU basis. Herein, the size ofthe ECU may be determined on the basis of a value transmitted through atleast one among a sequence, a picture, a tile, a slice (segment) header,and a parameter set, or may be derived using the size of the CTU, themaximum transform size, the size of at least one among a CU, a PU, a TU,a CB, a PB, and a TB, or the like.

Further, the encoding/decoding order of the ECUs may be the same as theencoding or decoding order of the coding units (CUs) having the samesize as the ECUs or of the transform units (TUs) within the CU. When theECU is defined on a per-subblock basis, the update order of the contextis the same as the entropy encoding or decoding order (scanning order)for the subblock.

As another example, the encoding or decoding order of the ECUs may bepredefined between the encoder and the decoder, or may be determinedaccording to a value transmitted through at least one among a sequence,a picture, a tile, a slice (segment) header, and a parameter set.

For example, the context may consist of variables that represent theprobability of occurrence of 0 or 1 for one bin, and may include a mostprobable symbol (MPS) or least probable symbol (LPS) binary value, and aprobability value (or an index representing the probability) therefor.Among binary strings for respective syntax elements, at least onecontext may be used for a particular bin and thus entropy encodingefficiency may be enhanced. In this specification, a bin usingprobability determined on the basis of a context may be defined as aregular bin, and a bin using fixed probability (for example, 0.5) may bedefined as a bypass bin. Further, during the encoding or decodingprocess, when the context is updated by applying a statisticalcharacteristic of the occurring binary value, the encoder or decoder hashigher encoding efficiency. Herein, the regular bin and the context binmay have the same meaning.

FIG. 14 is a diagram illustrating a context and a binary stringaccording to an embodiment of the present invention.

The table in FIG. 14 shows an example of a binary string of an x or yabsolute value (abs_mvd) syntax element of a motion vector differenceand an assigned context. As shown at the bottom of FIG. 14, contexts maybe assigned only for some binary indexes. Contexts may not be assignedfor the remaining bins, and entropy encoding or decoding may beperformed assuming that the probability of 0 or 1 is 0.5. When a contextis assigned for a binary string, each context includes probabilityinformation that is updatable on a per-ECU basis. The probabilityinformation may be represented as probability of 1 (or probability of0), probability of the MPS and LPS values (or probability of MPS), orthe like. For example, a probability value may be represented in a formin which an index value in the form of an integer value predefined bythe encoder or decoder is mapped to each bin.

For example, contexts may be updated using information on bins occurringwhen encoding or decoding previous ECUs of the current ECU, and theupdated contexts may be used when entropy encoding or decoding thecurrent ECU.

Herein, the context assigned to all syntax elements occurring whenencoding or decoding at least one previous ECU may be updated. Asanother example, only contexts assigned to bins included in the residualsignal-related syntax element may be updated.

For example, with respect to at least one of residual signal-relatedsyntax elements tu_cbf_luma, tu_cbf_cb, tu_cbf_cr, transform_skip_flag,tu_mts_flag, mts_idx, last_sig_coeff_x_prefix, last_sig_coeff_y_prefix,last_sig_coeff_x_suffix, last_sig_coeff_y_suffix, coded_sub_block_flag,sig_coeff_flag, par_level_flag, abs_level_gtx_flag, abs_remainder,dec_abs_level, and coeff_sign_flag for N (herein, N>=1) TUs or subblocksoccurring during encoding or decoding in the previous ECU, thecorresponding contexts may be updated before entropy encoding ordecoding is performed on the current ECU.

For example, when a flag (tu_cbf_luma, tu_cbf_cb, or tu_cbf_cr)indicating whether a residual signal to be encoded or decoded for any TUis present is 0, the flags are updated and context updates for theremaining residual signal-related syntax elements (transform_skip_flag,tu_mts_flag, mts_idx, last_sig_coeff_x_prefix, last_sig_coeff_y_prefix,last_sig_coeff_x_suffix, last_sig_coeff_y_suffix, coded_sub_block_flag,sig_coeff_flag, par_level_flag, abs_level_gtx_flag, abs_remainder,dec_abs_level, coeff_sign_flag, and the like) are omitted.

As another example, only contexts assigned to bins of syntax elementsexcept for syntax elements of a higher concept than the ECU may beupdated. For example, a partitioning flag syntax element may be a syntaxelement that indicates whether a block (the CTU when the ECU is based onthe CU or TU) which may be a higher concept than the ECU is partitioned.In this case, the context for binary values in which the context is usedin the partitioning flag syntax element may not be updated on a per-ECUbasis, but may be updated whenever encoding or decoding occurs. To thisend, the encoder and the decoder may predefine which context is alwaysupdated.

As still another example, in the case where the ECU is based on asubblock, when a context update for residual signal-related syntaxelements included in the subblock is omitted and an index for thesubblock is changed, among the residual signal-related syntax elementsincluded in the subblock, before performing encoding or decoding for thefirst syntax element, a context update is performed on the residualsignal-related syntax elements in the subblock. The other syntaxelements may be subjected to a context update on a per-bin basis.

For example, regarding syntax elements, among the residualsignal-related syntax elements included in the subblock, a contextupdate for at least one of syntax elements transform_skip_flag,tu_mts_flag, mts_idx, last_sig_coeff_x_prefix, last_sig_coeff_y_prefix,last_sig_coeff_x_suffix, last_sig_coeff_y_suffix, coded_sub_block_flag,sig_coeff_flag, par_level_flag, abs_level_gtx_flag, abs_remainder,dec_abs_level, and coeff_sign_flag, which use a regular bin, may beomitted, and the context update for the syntax element may be performedwhen encoding or decoding of the subsequent subblock starts.

As still another example, in the case where the ECU is a unit includinga predefined number of bins, a context update may be performed on thebasis of N regular bins.

For example, entropy encoding or decoding may be performed on N bins inthe current ECU without a context update. Before performing entropyencoding or decoding on the subsequent ECU, a context update may beperformed for syntax elements of the N bins. Further, for example,entropy encoding or decoding may be performed on N regular bins in thecurrent ECU without a context update. Before performing entropy encodingor decoding on the subsequent ECU, a context update may be performed forsyntax elements of the N regular bins.

As still another example, for the N regular bins in the current ECU, thecontext update is performed while entropy encoding or decoding isperformed. For regular bins exceeding N regular bins, the context updatemay not be performed. Herein, the regular bin may refer to a contextbin.

According to the embodiment of the present invention, the context updateis performed on a per-ECU basis, so that dependency on the contextupdate process and the entropy encoding or decoding process within oneECU may be removed. Therefore, after entropy decoding of all theprevious ECUs is completed, the decoder may use the decoded syntaxelement information and the binary value to update the context to beused for entropy decoding of the current ECU.

Further, by removing the dependency on the context update process andthe entropy encoding or decoding process, the encoder or decoder maysimultaneously perform entropy encoding or decoding of the current ECUand the context update for the subsequent ECU.

As still another example, according to another embodiment of the presentinvention, with respect to the bin of the ECU included in the currentblock, the context update is performed for each bin. With respect to thebins not included in the ECU, entropy encoding or decoding may beperformed without the context update.

Hereinafter, a detailed embodiment of the context update step will bedescribed in detail.

For example, for any syntax element, by using the binary value occurringwhen entropy encoding or decoding at least one previous ECU and thecontext before the update, the context update may be performed accordingto the occurrence order. The updated context may be used to performentropy encoding or decoding of the subsequent ECU.

FIG. 15 is a diagram illustrating a context update method using alook-up table according to an embodiment of the present invention. FIG.16 is another diagram illustrating a context update method using alook-up table according to an embodiment of the present invention.

FIG. 15 shows utilization in a method of performing a context update onthe basis of the occurrence order. The context update according to theembodiment may be performed on the basis of a look-up table. Forexample, in the case where the total number of contexts is numCtx, wherethe number of bins occurring in the previous ECU for each context isnumDecBin, where the value of each bin is binVal, and where the value ofthe MPS is valMps, the encoder or decoder may continuously updateprobability values by referring to probability update tables transIdxLpsand transIdxMps according to whether binary value is the LPS or the MPSin the order in which the binary values occur. By using the probabilityvalue pStateIdx in which the update is finally completed, the entropyencoding or decoding process of the current ECU may be performed.

FIG. 16 is a diagram illustrating actual probability values for a totalof 64 probability indexes of the tables transIdxMps and transIdxLps thatstore probability index values changed for each case, when the MPS andthe LPS have occurred.

For example, the encoder or decoder may store all the binary values foreach context which occur in the previous ECU, and then may perform thecontext update processes for all the bins simultaneously. As anotherexample, the encoder or the decoder may perform the context updateimmediately after the occurrence of each bin by using the binary valueoccurring in the entropy encoding or decoding process of the previousECU and the corresponding context information.

FIG. 17 is a diagram illustrating a context update method using a linearprobability update model according to an embodiment of the presentinvention.

FIG. 17 shows an example of pseudocode that may be utilized in anoccurrence order-based context update method using a linear probabilityupdate model. In the case where the total number of contexts is numCtx,where the number of decoded bins occurring in the previous ECU for eachcontext is numDecBin, and where the value of each bin is binVal, theencoder or the decoder may continuously update the probability by usinga linear probability model according to whether the binary value is theLPS or the MPS in the order in which the binary values occur. By usingthe probability value pStateIdx in which the update is finallycompleted, the entropy encoding or decoding process of the current ECUmay be performed.

According to the pseudocode in FIG. 17, the encoder or the decoder mayindependently perform context updates for all the bins occurring in theprevious ECU with respect to each probability model by using two linearprobability update models a and b having different update rates, whichare capable of deriving the probability index for the probability of 1.The encoder or the decoder may use the average value pStateIdx of thetwo finally-updated probabilities pOStateIdx and plStateIdx for entropyencoding or decoding of the current ECU.

Further, in the case where a bin less than a threshold value predefinedusing the number numDecBin of each context occurs, the encoder or thedecoder may set the context update rate to be fast. On the other hand,in the case where a bin equal to or greater than the threshold valueoccurs, the encoder or the decoder may set the context update rate to beslow. Based on this, the encoder or the decoder may adaptively adjustthe update rate of the context on a per-ECU basis, thereby enhancing thecompression efficiency.

For example, the encoder or decoder may store all the binary values foreach context which occur in the previous ECU, and then may perform thecontext update processes for all the bins simultaneously. As anotherexample, the encoder or the decoder may perform the context updateimmediately after the occurrence of each bin by using the binary valueoccurring in the entropy encoding or decoding process of the previousECU and the corresponding context information.

As another example, for any syntax element, by using the binary valueoccurring when entropy encoding or decoding at least one previous ECUand the context before the update, the context update may be performedaccording to the frequency of occurrence of 0 and 1 for each context. Byusing the updated context, entropy encoding or decoding of thesubsequent ECU may be performed. That is, on the basis of the differencein the frequency of the occurrence of 0 and 1 for the context, thecontext may be updated.

FIG. 18 is a diagram illustrating a context update method using alook-up table according to another embodiment of the present invention.

FIG. 18 shows pseudocode that may be utilized for a method of performinga context update on the basis of the frequency of occurrence. Thecontext update according to the embodiment may be performed on the basisof a look-up table. For example, in the case where the total number ofcontexts is numCtx, and where the numbers of encoded or decoded bins 0and 1 in the previous ECU for each context are numDecBin0 andnumDecBin1, respectively, the encoder or the decoder may derive thedifference difCount in the frequency of occurrence and a binary valuebinVal with the greater frequency of occurrence of 0 and 1. The encoderor the decoder may perform the context update using the deriveddifference in frequency and the look-up table and may derive a finalprobability value pStateIdx. The finally derived probability value maybe used for entropy encoding or decoding of the current ECU.

For example, in the case where in encoding or decoding of the previousECU, 0 using any context occurs 10 times and 1 occurs six times,considering that 0 occurs four more times than 1 occurs, the encoder orthe decoder may perform the context update considering the state where 0occurs four times for the context. The encoder or the decoder may usethe updated context for entropy encoding or decoding of the current ECU.That is, the encoder or the decoder may perform the context update onthe basis of the difference in the frequency of occurrence. The encoderor the decoder may derive the final probability value on the basis ofthe updated context for use in the entropy decoding process of thecurrent ECU.

FIG. 19 is a diagram illustrating a context update method using a linearprobability update model according to another embodiment of the presentinvention.

FIG. 19 shows pseudocode that may be utilized for a context updatemethod based on the frequency of occurrence using a linear probabilityupdate model. In the case where the total number of contexts is numCtx,and where among the encoded or decoded bins occurring in the previousECU for each context, the numbers of 0 and 1 are numDecBin0 andnumDecBin1, respectively, the encoder or the decoder may derive thedifference difCount in the frequency of occurrence and the binary valuebinVal with the great frequency of occurrence, and may put the resultsto the linear probability model to derive a probability value pStateIdxwith a single calculation. The derived probability value may be used forthe entropy encoding or decoding process of the current ECU.

In FIG. 19, the encoder or the decoder may update the context using twolinear probability update models a and b having different update rates,which are capable of deriving the probability index for the probabilityof 1. Herein, the encoder or the decoder may update the context assumingthat only bins with the great frequency in the previous ECU occur. Theaverage value pStateIdx of the two finally-derived probabilitiespOStateIdx and plStateIdx may be used for entropy encoding or decodingof the current ECU.

As another example, in the case where a bin less than a threshold valuepredefined using the frequency of occurrence for each context occurs,the encoder or the decoder may set the context update rate to be fast.On the other hand, in the case where a bin equal to or greater than thethreshold value occurs, the encoder or the decoder may set the contextupdate rate to be slow. Herein, the predefined threshold value may bedetermined using the sum of the frequencies of occurrence (the sum ofnumDecBin0 and numDecBin1) for each context. Based on this, the encoderor the decoder may adaptively adjust the update rate of the context on aper-ECU basis, thereby enhancing the compression efficiency.

For example, the encoder or the decoder may calculate the difference infrequency of binary values occurring in the previous ECU to perform thecontext update processes for all the bins simultaneously. As anotherexample, the encoder or the decoder may calculate the difference infrequency of binary values occurring in the entropy encoding or decodingprocess of the previous ECU, and may perform the context updateimmediately after the occurrence of each bin.

As still another example, the encoder or the decoder may derive aprobability distribution model similar to the probability distributionof bins of at least one previous ECU, and may use the derivedprobability distribution model to update at least one context forentropy encoding or decoding of the current ECU.

FIG. 20 is a diagram illustrating a context update method using aprobability distribution model according to an embodiment of the presentinvention.

As shown in FIG. 20, when the probability distribution for abs_mvd ofprevious ECUs is similar to 0-th order exponential Golomb (EGO), theencoder or the decoder changes the probability value of the context fortwo or more bins of abs_mvd by using the probability distribution P(x)optimum for the 0-th order exponential Golomb binarization. Herein, thecase where the abs_mvd probability distribution of previous ECUs issimilar to the 0-th order exponential Golomb (EGO) may refer to a casewhere it is determined that the probability of occurrence for each of 0and 1 is similar to the probability distributionP(x)=1/(2(x+1){circumflex over ( )}2) optimum for the 0-th orderexponential Golomb binarization. Further, the probability values of thecontexts for 0 and 1 may also be changed using the P(x). The encoder orthe decoder may perform entropy encoding or decoding for abs_mvd of thecurrent ECU by using the context in which the probability value ischanged. That is, the encoder or the decoder may define at least oneprobability distribution model used in any syntax element, may derivethe probability distribution model similar to the probabilitydistribution for the syntax element in the previous ECUs, and mayperform, on the basis thereof, the context update.

That is, the encoder or the decoder may update probability valuessimultaneously using the probability distribution of pieces ofpre-encoded or decoded binary information and a predefined probabilitydistribution model, thereby enhancing the throughput.

Hereinafter, the above-described entropy encoding or decoding step willbe described in detail.

According to an embodiment of the present invention, entropy encoding ordecoding may be performed on all blocks within the current ECU withoutthe context update.

Further, according to another embodiment of the present invention, forthe syntax elements of the ECU included in the current block, thecontext update may be performed for each syntax element and entropyencoding or decoding may be performed. For syntax elements not includedin the ECU, entropy encoding or decoding may be performed withoutperforming the context update.

For example, for the syntax elements other than the residualsignal-related syntax elements, the context update may be performedwithin the current ECU.

As another example, for the contexts corresponding to the syntaxelements except for the syntax elements of a higher concept than theECU, the context update may be performed within the current ECU.

The encoder or the decoder may use the binary value occurring in theentropy encoding or decoding process for the current ECU and the contextinformation to update the context to be used for encoding or decoding ofthe subsequent ECU. That is, the arithmetic entropy encoding or decodingprocess for the current ECU may be performed simultaneously with thecontext update process for the subsequent ECU.

FIG. 21 is a diagram illustrating a method in which an entropy encodingor decoding process and a context update process are simultaneouslyperformed according to an embodiment of the present invention.

FIG. 21 shows an example in which a process of updating the context on aper-ECU basis and an arithmetic entropy decoding process are implementedin a pipeline form. As shown in FIG. 21, the decoder may perform contextinitialization with a predefined value with respect to the first ECU ofthe picture, thereby performing entropy decoding. The decoder mayperform decoding on pixels within the ECU by using entropy decodedsyntax elements. For example, arithmetic entropy decoding and pixeldecoding may be performed on the basis of a CU within the ECU. Whileentropy decoding for the first ECU is performed, the context update forthe second ECU may be performed. Similarly, while arithmetic entropydecoding for the second ECU is performed, the context update for thethird ECU may be performed. Although the example in FIG. 21 is describedonly for the decoding process, the present embodiment may be equallyapplied to the encoding process. That is, the encoding steps forrespective ECUs may be simultaneously performed by the encoder.

As encoding or decoding of the current ECU and the subsequent ECU issimultaneously performed step by step, the throughput of the overallentropy decoding process may be increased.

Hereinafter, the above-described context initialization will bedescribed in detail.

The context initialization may be performed when the current ECU is thefirst ECU of a picture or slice. When the current ECU is the first ECUof a picture, slice, or tile, one or more predefined parameters are usedto initialize the context.

For example, by using a function f that may receive, as parameters, aquantization parameter QP for any context, a variation m of aprobability according to the quantization parameter, a probability nwhen the quantization parameter is 0, the initial probability P1 of bin1 for any context may be derived. Further, the probability of 0 (P0),the MPS, and the LPS may be derived using the probability of the bin 1.For example, the probabilities of bins 0 and 1, the MPS, and the LPS maybe derived according to Equation 1 below.

P1=f(QP,m,n),P0=1−P1

MPS=(P1>=0.5)?1:0,LPS=!MPS  [Equation 1]

The parameters for deriving an initial context probability may bedefined to have different values according to at least one among a slicetype, a type of image to be encoded or decoded, an application field,and a profile. According to this initial setting, initial context valuesappropriate for the characteristics, such as the type of image, theapplication field, the profile, and the like, may be used, and thus thecompression efficiency may be enhanced.

For example, in the case of a screen content image, the probability ofoccurrence of a skip mode is higher than that of an image of nature.Therefore, initial probability information for the skip mode may be setto a value higher than that of the initial probability used in the imageof nature.

When the probability distribution for any syntax element is similar to apredefined probability distribution model, the encoder or the decoderderives initial probability values for at least one context assigned toany syntax element by taking a binarization method predefined in thesyntax element into consideration.

FIG. 22 is a diagram illustrating context initialization according to anembodiment of the present invention.

For example, as shown in FIG. 22, when probability density distributionfor any syntax element abs_mvd is similar to the probabilitydistribution, which is P(x)=2{circumflex over ( )}(−(x−1)) (x>=0), ofthe unary binarization method, the probability density distributionfunction P(x) of the unary binarization method is used to performcontext initialization for N (N>0) binary values.

Equation 2 below is an example in which when the binarization method forabs_mvd is defined as shown in FIG. 14 above, the probability C0(i) of 0for the binary index(i) of 0 to 2 is derived using the probabilitydistribution model P(x).

C0(0)=P(0)=0.5

C0(1)=P(1)/P(0)=0.25/0.5=0.5

C0(2)=P(2)/P(1)=0.125/0.25=0.5  [Equation 2]

As another example, when the binarization method predefined for anysyntax element is unary binarization, the encoder or the decoder derivesthe initial values of all contexts used in the corresponding syntaxelement by using the probability distribution P(x)=2{circumflex over( )}(−(x−1)) (x>=0) of the unary binarization method.

In the meantime, when the current picture supports CTU column-basedparallel encoding and decoding using wave-front parallel processing(WPP), the ECU 0 in the CTU column 0 of the picture uses predefinedinitial context values to initialize all contexts. In the meantime, theECUs 0 within the CTU x of CTU column k (herein, k>0) may be subjectedto entropy encoding or decoding using the context used when the firstECU within the CTU y (herein, y>x) of the CTU column k−1 is entropyencoded or decoded.

FIG. 23 is a diagram illustrating context initialization according toanother embodiment of the present invention.

FIG. 23 shows the CTU column-based parallel entropy encoding or decodingmethod using the WPP to which context initialization according to thepresent invention is applied. As shown in the figure, the ECU 0 of theCTU column 1 may be subjected to entropy encoding or decoding using thecontext used for entropy encoding or decoding of the ECU 8 of the CTUcolumn 0 that is a higher CTU column. That is, the encoder or thedecoder may update the context using at least one among the number ofoccurrences of bins within the syntax element which occur when entropyencoding or decoding the ECU 7 of the CTU column 0, and a variableindicating the difference between the number of occurrences of 0 and thenumber of occurrences of 1. The updated context may be used for entropyencoding or decoding of the ECU 8 of the CTU column 0 and the ECU 0 ofthe CTU column 1.

Hereinafter, a syntax element grouping method will be described.

FIG. 24 is a diagram illustrating a bitstream according to an embodimentof the present invention. FIG. 25 is a diagram illustrating a bitstreamaccording to another embodiment of the present invention.

The bitstream in FIG. 24 may be an example of the order of generalsyntax elements within a bitstream. As shown in FIG. 24, in order toperform entropy decoding and reconstruction on a per-CU basis, allsyntax elements included in each CU need to be transmitted in theencoding or decoding order according to each CU. Herein, when the CU ispartitioned into one or more TUs, several pieces of TU information inthe CU are encoded or decoded in order. Since various syntax elementsmay be present within the CU, changes of the context may occurfrequently during entropy encoding or decoding. Therefore, the contextneeds to be frequently loaded from the external memory, and thus thethroughput in entropy encoding or decoding by the encoder and thedecoder may be reduced.

The bitstream in FIG. 25 may be an example of improving the bitstream inFIG. 24. According to an embodiment of the present invention, the syntaxelement may be encoded or decoded on a per-ECU basis. The ECU mayconsist of at least one CU, so that the same syntax elements of the CUsincluded in the ECU may be grouped.

FIG. 25 shows a bitstream generated by performing ECU-based syntaxelement grouping. For example, when two CUs are present within one ECU,the encoder groups two pred mode syntax elements for each CU and thengroups two MVDs for each CU. Afterward, the encoder may group andtransmit syntax elements for residual signal information. In order todecode the bitstream composed of ECU-based syntax element groups, thedecoder may separate the grouped syntax elements into CU-based syntaxelements.

For example, the encoder or the decoder may perform ECU-based groupingonly for residual signal information-related syntax elements. Forexample, for the residual signal information-related syntax elementsthat may consist of at least one syntax element, the encoder or thedecoder may perform ECU-based grouping on the same syntax elements andmay perform encoding or decoding. For example, when four TUs are presentwithin one ECU, the encoder or the decoder constructs groupssig_coeff_flag, gt1_flag, gt2_flag, and remain_abs_level for the fourTUs and performs encoding or decoding.

According to the present invention, the frequency of changes of thecontext may be reduced, so that the throughput of entropy encoding ordecoding may be

The above embodiments may be performed in the same method in an encoderand a decoder.

At least one or a combination of the above embodiments may be used toencode/decode a video.

A sequence of applying to above embodiment may be different between anencoder and a decoder, or the sequence applying to above embodiment maybe the same in the encoder and the decoder.

The above embodiment may be performed on each luma signal and chromasignal, or the above embodiment may be identically performed on luma andchroma signals.

A block form to which the above embodiments of the present invention areapplied may have a square form or a non-square form.

The above embodiment of the present invention may be applied dependingon a size of at least one of a coding block, a prediction block, atransform block, a block, a current block, a coding unit, a predictionunit, a transform unit, a unit, and a current unit. Herein, the size maybe defined as a minimum size or maximum size or both so that the aboveembodiments are applied, or may be defined as a fixed size to which theabove embodiment is applied. In addition, in the above embodiments, afirst embodiment may be applied to a first size, and a second embodimentmay be applied to a second size. In other words, the above embodimentsmay be applied in combination depending on a size. In addition, theabove embodiments may be applied when a size is equal to or greater thata minimum size and equal to or smaller than a maximum size. In otherwords, the above embodiments may be applied when a block size isincluded within a certain range.

For example, the above embodiments may be applied when a size of currentblock is 8×8 or greater. For example, the above embodiments may beapplied when a size of current block is 4×4 only. For example, the aboveembodiments may be applied when a size of current block is 16×16 orsmaller. For example, the above embodiments may be applied when a sizeof current block is equal to or greater than 16×16 and equal to orsmaller than 64×64.

The above embodiments of the present invention may be applied dependingon a temporal layer. In order to identify a temporal layer to which theabove embodiments may be applied, a corresponding identifier may besignaled, and the above embodiments may be applied to a specifiedtemporal layer identified by the corresponding identifier. Herein, theidentifier may be defined as the lowest layer or the highest layer orboth to which the above embodiment may be applied, or may be defined toindicate a specific layer to which the embodiment is applied. Inaddition, a fixed temporal layer to which the embodiment is applied maybe defined.

For example, the above embodiments may be applied when a temporal layerof a current image is the lowest layer. For example, the aboveembodiments may be applied when a temporal layer identifier of a currentimage is 1. For example, the above embodiments may be applied when atemporal layer of a current image is the highest layer.

A slice type or a tile group type to which the above embodiments of thepresent invention are applied may be defined, and the above embodimentsmay be applied depending on the corresponding slice type or tile grouptype.

In the above-described embodiments, the methods are described based onthe flowcharts with a series of steps or units, but the presentinvention is not limited to the order of the steps, and rather, somesteps may be performed simultaneously or in different order with othersteps. In addition, it should be appreciated by one of ordinary skill inthe art that the steps in the flowcharts do not exclude each other andthat other steps may be added to the flowcharts or some of the steps maybe deleted from the flowcharts without influencing the scope of thepresent invention.

The embodiments include various aspects of examples. All possiblecombinations for various aspects may not be described, but those skilledin the art will be able to recognize different combinations.Accordingly, the present invention may include all replacements,modifications, and changes within the scope of the claims.

The embodiments of the present invention may be implemented in a form ofprogram instructions, which are executable by various computercomponents, and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include stand-alone or acombination of program instructions, data files, data structures, etc.The program instructions recorded in the computer-readable recordingmedium may be specially designed and constructed for the presentinvention, or well-known to a person of ordinary skilled in computersoftware technology field. Examples of the computer-readable recordingmedium include magnetic recording media such as hard disks, floppydisks, and magnetic tapes; optical data storage media such as CD-ROMs orDVD-ROMs; magneto-optimum media such as floptical disks; and hardwaredevices, such as read-only memory (ROM), random-access memory (RAM),flash memory, etc., which are particularly structured to store andimplement the program instruction. Examples of the program instructionsinclude not only a mechanical language code formatted by a compiler butalso a high level language code that may be implemented by a computerusing an interpreter. The hardware devices may be configured to beoperated by one or more software modules or vice versa to conduct theprocesses according to the present invention.

Although the present invention has been described in terms of specificitems such as detailed elements as well as the limited embodiments andthe drawings, they are only provided to help more general understandingof the invention, and the present invention is not limited to the aboveembodiments. It will be appreciated by those skilled in the art to whichthe present invention pertains that various modifications and changesmay be made from the above description.

Therefore, the spirit of the present invention shall not be limited tothe above-described embodiments, and the entire scope of the appendedclaims and their equivalents will fall within the scope and spirit ofthe invention.

INDUSTRIAL APPLICABILITY

The present invention may be used to encode or decode an image.

1. A method of decoding an image, the method comprising: determiningwhether to perform a context update for a first syntax element of acurrent block; updating, on the basis of the determination, a contextfor entropy decoding of the first syntax element; and generating, on thebasis of the updated context, a bin for the first syntax element,wherein whether to perform the context update is determined on the basisof the number of pre-decoded predetermined syntax elements for thecurrent block.
 2. The method of claim 1, wherein when the number of thepre-decoded predetermined syntax elements is equal to or less than apreset value, the context update for the first syntax element isperformed.
 3. The method of claim 2, wherein the preset value isdetermined on the basis of a size of the current block.
 4. The method ofclaim 1, wherein only when the current block is in a transform skipmode, the context update is performed.
 5. The method of claim 1, whereinthe first syntax element is a syntax element related to a residualsignal of the current block.
 6. The method of claim 5, wherein the firstsyntax element is the syntax element indicating a sign of a quantizedlevel.
 7. The method of claim 6, wherein the first syntax element iscoeff_sign_flag.
 8. The method of claim 1, wherein the predeterminedsyntax element is at least one of residual signal-related syntaxelements.
 9. The method of claim 7, wherein the predetermined syntaxelements are coeff_sign_flag, sig_coeff_flag, abs_level_gtx_flag, andpar_level_flag.
 10. A method of encoding an image, the methodcomprising: generating a bin for a first syntax element of a currentblock by binarizing the first syntax element; determining whether toperform a context update for the first syntax element; updating, on thebasis of the determination, a context for entropy encoding of the firstsyntax element; and generating a bitstream for the current block byusing the updated context and the generated bin, wherein whether toperform the context update is determined on the basis of the number ofpre-encoded predetermined syntax elements for the current block.
 11. Themethod of claim 10, wherein when the number of the pre-encodedpredetermined syntax elements is equal to or less than a preset value,the context update for the first syntax element is performed.
 12. Themethod of claim 11, wherein the preset value is determined on the basisof a size of the current block.
 13. The method of claim 10, wherein onlywhen the current block is in a transform skip mode, the context updateis performed.
 14. The method of claim 10, wherein the first syntaxelement is a syntax element related to a residual signal of the currentblock.
 15. The method of claim 14, wherein the first syntax element isthe syntax element indicating a sign of a quantized level.
 16. Themethod of claim 15, wherein the first syntax element is coeff_sign_flag.17. The method of claim 10, wherein the predetermined syntax element isat least one of residual signal-related syntax elements.
 18. The methodof claim 17, wherein the predetermined syntax elements arecoeff_sign_flag, sig_coeff_flag, abs_level_gtx_flag, and par_level_flag.19. A computer-readable recording medium storing a bitstream that isreceived by an apparatus for decoding an image and is used toreconstruct a current block included in a current picture, wherein thebitstream is generated by a method of encoding an image, the methodincluding: generating a bin for a first syntax element of the currentblock by binarizing the first syntax element; determining whether toperform a context update for the first syntax element; updating, on thebasis of the determination, a context for entropy encoding of the firstsyntax element; and generating a bitstream by using the updated contextand the generated bin, wherein whether to perform the context update isdetermined on the basis of the number of pre-encoded predeterminedsyntax elements for the current block.